Fork Oil?

[KJL316]

I have a bad fork seal so I will be replacing them both IF it ever warms up!!! According to the service manual for my 1978 GS550, use 50/50 ATF and 10W/30 motor oil. Which type of ATF and standard vs synthetic oil? This would be a little cheaper than buying pre-mixed fork oil but does anyone have an opinion as to which is the better alternative? Thanks!

 

[Nessism]

10 or 15 weight fork oil is what most use. Depends on how firm you like the ride and your weight.

 

[cowboyup3371]

Just buy regular fork oil and not worry about it

 

[Steve]

"Pre-mixed fork oil"??? :-k

That's like looking for "pre-mixed engine oil". :-\\\

I read some suspension tuning tips from the manager of a race team several years ago. His philosophy for the race bike was to use the lightest oil that would damp the suspension just enough to keep the wheel on the ground. 10w fork oil seems to work for most of us.

While you have the forks apart, consider upgrading the springs, too. Your springs are now 40 years old and have been compressed the whole time, so I'll bet they are sagging a bit more than desired. A set of new Sonic Springs and the 10w oil will make you think you have a new bike under you.

.

 

[Brendan W]

I'm old school and mix atf with whatever 10/40 or 15/40 I am using for the motor and whatever atf is lying around. The forks had a lot of sludge so it suited me to change often for the first few years of ownership.
This gave me a chance to experiment and my target now is around 15W effective.
ATF has a bigger effect than you think on thinning the motor oil. There is a lot of stuff on the web about mixing different viscosities.
If I remember correctly you would typically use 2:1 engine oil to atf if your target was halfway between the two.
The viscosities on atf bottles are not directly comparable with motor oil weights so a bit of research is needed to be sure you're comparing apples with apples.

 

[Downs]

I just run straight ATF. Feels fine to me.