# Which Fork Oil? Regular or Semi Synthetic?



## roig12 (Jul 26, 2014)

What is the difference between regular and semi synthetic?


Roy


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## charging_rhinos (Jul 29, 2008)

Semi-synthetic is just that: mixed regular and synthetic oils. You might think of synthetic as a higher-end premium oil. Oils have things called mer chains in them, which are in essence long strings of molecules. We usually call them polymer chains. As the oil moves/flows, these 'chains' of molecules all slide past one another. The length of these chains determines how easily they slide. You might be able to imagine that a longer chain of these molecules would be harder to slide past one another than little, short chains.

When oils, and this is really simplifying it mind you, are 'thicker' or have a higher 'weight,' they have longer polymer chains. When regular oil is refined and these chains are measured, they vary in size quite a bit. It's the average viscosity or weight of the oil that's measured, so there can be lots of overly long chains and overly short chains in a bottle of oil, as long as the average is at the desired number. 

As oil is used, these molecular chains break. All the friction, pressure, being forced through valves, etc., breaks these long chains into smaller chains. This lowers the viscosity or thickness of the oil. That's partly why old, used-up oil in cars is much more watery when you drain it than the thick, new stuff you put in. So if you begin with oil with a bunch of short chains mixed in with long chains, you might imagine that it would take less time to break down the long chains, as there are fewer of them to break. So the oil becomes thinner faster.

In the creation of synthetic oils, the process is far more controlled. This is arguable, but the theory still holds. As a result of these more controlled chemical processes, the length of these polymer chains is more consistent. This means that you've got less short chains to start with. Therefore, the oil can usually last a bit longer before it breaks down to the point that it is not viscous enough to protect the moving parts. 

Now, after all that word vomit, let's tie it all in with mountain bikes: It probably doesn't matter too much which oil you choose. It takes a lot of heat, pressure, and friction to really break oil down to the point that your forks are wearing down. Think about the abuse a car's engine deals out, versus that of a fork. Not even in the same ballpark. The biggest issue with bikes is contamination. Dirt gets past the seals and into the oil. That oil flows around, passing the dirt through your valves, bushings, etc, scratching them up. The oil is still fine, but the particulate is damaging everything. So basically, your oil won't get trashed due to viscosity break-down. It'll need to be changed because you ride in the dirt. So for me, I just get good quality regular fork oil (Maxima) and change it out at least 2-3 times per season.

Sorry for the length of the post, but I am a firm believer that people should be educated and intelligent beings.


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## roig12 (Jul 26, 2014)

Thanks alot oil man! Learned alot from your detailed explanation!
Going to order my oil now 


Thanks again,
Roy


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## Le Duke (Mar 23, 2009)

Charging Rhinos made some great points.

Another to consider is the weight of the oil to be used, which can have some implications for the damper's performance, as well as it's ability to perform at different temperatures.

While I'm just an XC weenie, the DH guys at my local shop got me to change my oil weight in response to changing temps. Heavier weights for hotter temperatures, lighter weights for cooler temps. Maybe it's just in my head, but I can feel the difference.


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## csermonet (Feb 2, 2009)

I would just order what is recommended from the manufacturer, suspension oil is not something you want to experiment with if you don't know what you are doing. trust me, ask me how i know


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## charging_rhinos (Jul 29, 2008)

Le Duke said:


> Charging Rhinos made some great points.
> 
> Another to consider is the weight of the oil to be used, which can have some implications for the damper's performance, as well as it's ability to perform at different temperatures.


^ This. Great advice. I also change my oil weight according to the temperature. I don't go outside with a thermometer before each ride or anything. But during cold months I'll usually drop the viscosity by 5 (ex: from 15w down to 10w) to counteract the added thickness of the cold oil. I can definitely feel a difference in the damping. YMMV.


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## roig12 (Jul 26, 2014)

Just ordered a 5w and a 15w as I need both...I wouldn't buy just one weight to save $15 so yeah going by the service manual I've downloaded for my Boxxer


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## Le Duke (Mar 23, 2009)

charging_rhinos said:


> ^ This. Great advice. I also change my oil weight according to the temperature. I don't go outside with a thermometer before each ride or anything. But during cold months I'll usually drop the viscosity by 5 (ex: from 15w down to 10w) to counteract the added thickness of the cold oil. I can definitely feel a difference in the damping. YMMV.


Exactly what I do. 15w to 10w.

I wonder if WC DH mechanics do this based on the tracks, too. It would make sense to use different weights; i.e., a course with a boat load of smaller hits vs. a shorter track with fewer but larger ones.

It would make sense, but those guys are the wizards, and understand this stuff a hell of a lot better than I do.


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## SHIVER ME TIMBERS (Jan 12, 2004)

Golden Spectro is a great oil to use in Forks


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## csermonet (Feb 2, 2009)

SHIVER ME TIMBERS said:


> Golden Spectro is a great oil to use in Forks


I love comments like this. An oil with the same properties as the manufacturer recommended is the only good oil. There are some great oils out there that aren't manufactured with the required properties that's recommended for said fork/shock, and that makes them **** oils.


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## RWhiz (Jun 10, 2013)

roig12 said:


> What is the difference between regular and semi synthetic?
> 
> Roy


To answer the question, oils are made up of a base oil and an additive package. For this discussion we are talking about the base oil. First read Charging Rino's post, it has some good info. There are basically 5 groups of base oils, groups 1 and 2 are standard base oils derived from crude oil and refined to different degrees. Group 3 is hydro cracked crude based oil, however it can be referred to as full synthetic oil as companies have sued for this naming structure. It is regular oil modified for better performance. Group 4 is a real full synthetic oil made from POA. Not sure the chemical that POA refers to however it is very strong oil. It was developed for fighter plane engines and Amsoil uses this as a base for all or most of their oils. Group 5 oils are easily the strongest, most protective, as well as having other good characteristics such as bonding to metals. Redline and Motul are popular brands which use group 5 oils. These are made from POE's or poly esters and are often referred to as ester based oils. These were developed for use in jet engine bearings. My favorite suspention oil uses POE's, it's Motorex. I've used it in MX suspention for years and really feel the difference. Often manufacturers will specify a type of oil to use in their forks, the quality of oil is up to you to buy and put into your equipment though.


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## Le Duke (Mar 23, 2009)

Great info, RWhiz.

Do you think seal brand (SKF vs. stock RS vs. Enduro, for example) has any bearing on the type of oil, or weight, to be used?

Put another way, do some seals work better with a certain oil brand, oil weight, or temperature range? 

I'm an XC racer, but this stuff intrigues me to no end. I'm if dropping $7k on a bike, I want the squishy parts to work as well as possible. I recognize that the DH guys are the resident suspension gurus, thus my questions here.

Sent from my XT1585 using Tapatalk


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## Varaxis (Mar 16, 2010)

Le Duke said:


> Great info, RWhiz.
> 
> Do you think seal brand (SKF vs. stock RS vs. Enduro, for example) has any bearing on the type of oil, or weight, to be used?
> 
> ...


Petroleum-based oil eats rubber. They could make the seals out of a material like polyurethane to prevent that, but it could be that having some form of rubber is a more economical choice and using a fully synthetic oil is the only way to ensure the fork's seals do their job over the long term.

When I serviced my Pike, I used Redline Extra Light Blue for the charger damper and Maxima Ultra 0w-30 for the fork leg bath.

I use Fox's recommended branded stuff for their products, though I did substitute Mobil1 for fork leg bath before Fox switched to the 20wt gold.


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## RWhiz (Jun 10, 2013)

Historically group 4 oils have had some negative impact on seals. I believe that seal technology and modifications to the group 4 oils have negated the leaking seal issues from when group 4 oils were first put on the market. That was back in the '70's so cars built with older seals were effected, and for some time while manufacturers correct the issue. That's given synthetic oils a bad rep, however I believe that is all behind us now. I haven't had an issue with seals in cars, MX forks, MX shocks, or mountain bike forks using synthetic oils. I've never used group 4 oils though, only group 3 (cars) and group 5 (sports cars, MX bikes and mountain bike suspensions) Hope this helps. 

One more thing, when buying group 5 oils, prepair for an eye watering-ly expensive experience!


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## SHIVER ME TIMBERS (Jan 12, 2004)

csermonet said:


> I love comments like this. An oil with the same properties as the manufacturer recommended is the only good oil. There are some great oils out there that aren't manufactured with the required properties that's recommended for said fork/shock, and that makes them **** oils.


alcohol may or not been involved....but I don't understand what you are trying to say

BTW....only have rode marz and DVO Suspension which is Golden Spectro


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## csermonet (Feb 2, 2009)

I should have been more specific. If a manufacturer recommends Golden Spectro 5wt in the damper, Redline 5wt is not necessarily a suitable replacement. You need to look at the individual oil's viscosity index and reported centistokes rating at a given heat, specifically the cSt at 40c. They also give a rating at 100c but this is less important because a mountain bike damper will rarely if ever reach 100c. If a given oil in 5wt flavor is recommended by the manufacturer, the fork will perform as designed. But using a different brand's 5wt may cause your fork to feel like no amount of knob turning makes a noticeable difference in feel. NEVER look solely at the "wt" rating, ALWAYS look for comparable VI and CST values. Now if you are talking about bath oil, I personally use Fox Green in all my forks(Fox and RS). It is designed to be a bath oil, with an additive package that adds to the "slipperyness". Specifically formulated to do its job of lubricating stanchions moving through the fork bushings. Using a different oil can very quickly wreck the stanchions on a Fox fork. Not all suspension components are created equal though, so YMMV. My opinion is to always use what is recommended by the manufacturer unless you are a suspension guru that understands the effect of differing oil properties, which I readily admit that I am not. I got my information from the PVD Suspension Fluid page, link below. Here is an excerpt from that site:

DO NOT GO BY LABLED OIL WEIGHT! Not only is this a poor way to decide which oil to use, but each manufacturer seems to be on a different scale. For example Maxima RSF 7wt is actually lighter than Spectro 5wt and Silkolene Pro RSF 7.5wt is actually heavier than Showa SS-8 10wt. This is not a judgment about the quality of these oils, just that the 'weight' label leads to a lot of trouble when trying to tune with suspension oils.

Suspension oil is labeled by weight because consumers are used to thinking of oil (ie. motor oil & gear oil) in these terms. The SAE weight system has a very broad and vague viscosity range and does not even cover the viscosity range that most quality motorcycle suspension systems require. Another point of confusion is that motor oils and gear oils are rated for viscosity on the SAE scale at different temperatures, leading to similar viscosity oils having very different ratings. SAE J300 is used to define engine oils, while SAE J306 is used to define drive line (gear) and chassis lubricants.

Link to site if you or anyone else want to learn more - Suspension Fluid - Pvdwiki


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## Le Duke (Mar 23, 2009)

Great info, again. Seriously. Might be the most informative post I've read on MTBR this year.

Sent from my XT1585 using Tapatalk


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## RWhiz (Jun 10, 2013)

I read the link you provided in the post. Very good information in there.


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## Lelandjt (Feb 22, 2008)

For fork lubrication oil (air side leg and damper leg in closed damper forks) our shop has switched to Mobil 1 5w30 (maybe 10w30, can't remember) based on a recommendation from a suspension tuner. It just feels slipperier than others and is cheap. When Fox gold 20w came out I was all into the idea that its stickiness would keep it up on the seals and work better but then I switched to Mobil and immediately felt the difference. For the damper I always use what comes stock, then use valving and clickers to tune. Changing the oil thickness adds another variable to the already complex job of tuning.


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## mikesee (Aug 25, 2003)

Le Duke said:


> Charging Rhinos made some great points.
> 
> Another to consider is the weight of the oil to be used, which can have some implications for the damper's performance, as well as it's ability to perform at different temperatures.
> 
> While I'm just an XC weenie, the DH guys at my local shop got me to change my oil weight in response to changing temps. Heavier weights for hotter temperatures, lighter weights for cooler temps. Maybe it's just in my head, but I can feel the difference.


Not in your head at all. I go to 0w synthetic in winter, as anything else more or less freezes and renders the suspension non-functional.


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## mykel (Jul 31, 2006)

For a good synthetic have a hard look at Redline.
Highly regarded, Fully bendable. Been using it for years in all my forks.
Gets referenced quite a bit if you troll thru the peter verdone wiki site.

bit hard to find in Canukistan but possible for those who care.


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