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Difference between fork oil and motor oil?

  • Thread starter Thread starter t3rmin
  • Start date Start date
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t3rmin

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Can anyone tell me what the difference between fork oil and motor oil is? Why couldn't you just use plain old single-weight motor oil (without additives) in your forks? Is buying "fork oil" branded oil just a ploy to get you to pay more or is there really something to it?
 
fluid

fluid

It made to do that job. Its doesnt air rate and you can buy normaol that thin any way unless you use ATF (automatic transmission fluid) which you can do if you want to penny pinch.
 
Your seals will not work on engine oil due to it is a petroleum based oil, fork oil and trans fluid are a mineral based lubricant and the seals are made to hold those properties in.
 
Do you just have a bunch of single-weight oil sitting around with nothing to do, or are you just trying to save a bit of money?

I have not priced single-weight oil lately, but I imagine that it's about $2/quart like all the rest. The last time I got fork oil, I think it cost me about $15 to get enough to do the bike. The difference is not that great.

Motor oil is great at being pumped through the engine and keeping moving surfaces separated by a thin film under pressure. The additives are great for keeping combustion by-products in suspension until the next oil change. The last time I looked inside my forks, there was no oil pump, and definitely NO combustion, let alone by-products.

Fork oil is available in a greater variety of weights (viscosities) than most motor oils, making it far easier to tune the suspension performance. The bushings and seals are designed with fork oil in mind, why subject them to what is essentially a foreign substance and expect them to work properly?

You might save $15 or so the first time you try it, but if you have to go back and change a bunch of parts, it's going to cost you a lot more for parts, not to mention the time and aggravation.


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My old T305 from years ago recommended replacing the oil with 10-20-30 weight motor oil. Even in the late '60s and early '70s that was a difficult oil to obtain. I think I went with 10-30 instead. I know that's contradictory to the advice already offered but it is what Suzuki recommended. Still I would use something like PJ-1 fork oil. It has an additive to swell the seals and keep them from leaking.
 
My Suzuki service manual recommends 15w40 for some straight 30 for some and odd weights for others. It really has to do with the performance level of the bike and you can put whatever you want to a certain degree, trying to get the smoothest ride to your preference. if you change it as recommended or sooner, You'll have no oil related problems.
 
The Suzuki factory service manual for my GS550 calls for equal parts 10W/30 MOTOR OIL and ATF. Let this be your guide.
 
Nessism said:
The Suzuki factory service manual for my GS550 calls for equal parts 10W/30 MOTOR OIL and ATF. Let this be your guide.
Click to expand...

Yeah I did a bit more looking around and it seems like (some) fork oil is regular motor oil with anti-foaming additives. I suspect dino juice or dino juice + ATF would work just fine though.
 
You won't hurt your forks as long as there's some sort of oil in there.

As mentioned above, fork oil is just single viscosity oil with maybe a bit more anti-foaming additives. There are probably some anti-wear components in there to help deal with the sliding loads, and I'd bet there are plenty of detergents to help disperse and deal with all the muck and bug parts that get past the seals. You wouldn't want to put fork oil in your engine, but engine oil or transmission fluid won't hurt your forks.

But it's important to remember that the factory fork oil recommendations (and the impossibly soft stock fork springs) were chosen based on the experiences of very small Japanese test riders riding brand new motorcycles.

If you weigh more than 130 pounds and if your GS is not brand new, you will have much better results with a somewhat thicker fork oil.

I weigh about twice as much as the average resident of Hamamatsu, I ride very hard on really crappy Indiana back roads, and my GS has about 80,000 miles on it. So I'm probably an extreme case.

But after some experimenting, I have found that a mixture of 90% 15W fork oil and 10% "motor honey" (the thick clear yellowish goop sold as a motor oil "enhancer" for old, tired engines) works very well for me. The motor honey thickens the oil for more damping, but it also reduces stiction noticeably.

I would encourage others to experiment as well, keeping in mind that the factory recommendations should be just a starting point. You're not going to hurt your forks. Just a teaspoon or two of motor honey makes a noticeable difference, and can be a great way to fine tune damping. Or you may find your best results with a certain blend of motor oil and ATF. Find whatever works for you.

The same applies to fork springs -- the original worn out springs should be discarded ASAP, of course, but even with Progressive springs in place, you may still need to experiment with preload spacers to set sag correctly, especially if you are lighter or heavier than average.
 
A trick I've used successfully if you have a leaky seal; use a mix of transmission fluid with 'stop leak' (seal sweller), and compatible tranny fluid. You'll have a bit of leakage till the seal becomes more flexible. Consider this a temporary fix. 8-[

It's been a while back, but a guy on here wanted to take a fairly long trip on his GS, but a fork seal started leaking. He tried this 'fix' and was able to make the trip no problems. Seal quit leaking after a day or so.

As far as fork fluid, motor oil, transmission fluid, I've used them all. Right now in both my bikes I have Mobil One motor oil, 15W50 in the Kwacker and 5W30 in the Zook.
 
I use a blend of 20wt motor oil and atf...the factory recommended a blend of 10w30 and atf, or aft alone,
 
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