fork oil or sae 10-20 oil ?

I sent out my fork bottoms to get chromed, & will be getting them back soon, & just wanted to know if there is a difference between using fork oil or sae 10 20 oil. the service manual says the 10-20 oil. is one better than the other ?? Its for a 1980 LTD 1000

I'm running ATF in my 900. It's pretty much equivalent to 10w suspension fluid, and is a crapload cheaper.

What manual do you have that calls for motor oil in your forks?? I wouldn't put motor oil in my forks or fork oil in my motor. I use 15w Bel Ray Fork Oil in my forks, have done so for 34 years with no problems. Ed

Yes I am also running synthetic Valvoline ATF, and I add Lucas "Transmission Fix" to increase the viscosity to taste. I like my forks a bit stiffer.

kawasaki KZ 1000 service manual, fifth edition (1981), page 129

what ratio ATF to the trans fix do you use ? I"d like my forks a little stiffer also !....P,S. dont tell me (thats what she said )

While I prefer using a dedicated "fork" oil of whatever weight desired, and have also experimented using ATF,

The original 1973 Z1 OWNER'S MANUAL does indeed specify SAE 10W motor oil.


FSM for "73 Z1 ~ '76 KZ900 (Revised 6:Mar. 28,1977):
@ page 113: SAE 10W non-detergent
@ page 175: SAE 10W non-detergent

FSM for "77~79 KZ1000 (Revised 4:Mar. 12, 1979):
@ page 127: SAE 10W fork oil
@ page 183: SAE 10W
@ page 250: SAE 10W20
@ page 256: SAE 10W20

Am guessing the dedicated fork oil or ATF might enhance life of rubber seals.
Might be wrong, but think that "foaming" isn't an issue with motor oil, fork oil or ATF.
I do know the '73 Z1's original factory installed "fish oil" was nasty stuff.:pinch:

Good Fortune!

I think you don't want motor oil in the forks. Fork oil is more like hydraulic fluid than motor oil and has seal swell additives. I have heard ATF can work but it's viscosity is too thin for most applications (like mine) that call out 15W fork oil. I read that ATf viscosity is roughly like 7W fork oil. I do know thw SAE viscosity ratings for motor oil and fork oil are different. ATF will probably work but the best thing is synthetic fork oil since it's viscosity is very stable with temp change.

i would just use 20 fork oil. the damping in these old forks is almost none existant.

I have not been able to find fork oil any heavier than 20W. Now that being said, It must be realized that 20W fork oil is more viscous than 30W motor oil. Viscosity in SAE #s is not linear and it is not the same between motor, gear and hydraulic oils. Hydraulic oils are the heaviest/SAE# and gear oils are the lightest/SAE# Engine oils fall somewhere in between them. Just a rough estimate in equal viscosity would be something like this Hydraulic-15w SAE = Motor-30w SAE = Gear-83W SAE. These #s are approximate but close. You never want to use a detergent Motor oil in a hydraulic situation. ATF would be equivalent to about, 5W Hydraulic, 10W motor oil, and 70W gear oil. Not all ATFs are the same. By the way, fork oil falls into the hydraulic category.

I don't know where you folks are getting your info on hydraulics, but many hydraulic systems use motor oil instead of hydraulic oil, some even specify 10w40 motor oil, although probably most say 10w motor oil or SAE 32 hydraulic oil.

If it's a gear pump. then motor oil is fine in the hydraulic system, if it's a variable displacement pump then hydraulic oil only.

We are talking about fork oil. Fork oil is a high viscosity hydraulic oil. 10W motor oil would be about 5W hydraulic oil. I would normally use the ISO scale where 20W SAE would be about ISO 150. Like you said, most hydraulic systems don't care but none of them want detergent additives. but we are getting off the subject and too complicated for the base question.