Fork Oil

[tbeiler]

86 GL1200.
Got new oil seals, and bushings ready for install into the forks. The right fork developed a little leak last season.
Owners manual shows to use ATF as fork oil.
Looking up specs for ATF fluid, viscosity @ 40C, 28.7, viscosity @ 100C, 5.9, viscosity index, 154.
Apparently this equates to approx. a 7.5 wt fork oil.
I am running progressive springs in the front, and am not a heavy rider, rarely two up.
I understand that when running progressive front springs you can use a heavier oil, perhaps a 10W or 15 W.
A bit of research shows many different fork oils out there with a big range of viscosity indexes.

Anyone running progressives, (no shims), and what oil weight and brand are you using. Synthetic or dino?
I don't want to go too heavy, just don't know where to start as a baseline.
Previous owner put the the progressives in and the old oil that was drained out was not red in color, so I'm assuming it was not ATF.
Any advice is appreciated.

[golden highway]

I tried several different weights of fork oil and ATF. I found ATF worked the best. When I did the test I expected one of the fork oils to come out on top but ATF was far superior.

[tbeiler]

What weights of oil did you try, what didn't you like with the other weights and how did it affect your ride?
What made you choose to go back to the ATF?
Are you running progressive rate springs or do you have the standard springs installed?

[julimike54]

I've been running the Progressive springs for a while. I normally ride 2 up and prefer a "sporty ride". I followed the install instructions, see below....

Progressive instructions state to measure fluid level (max. 5.5" from top of fork, no spring installed), anything below that they show 'safe'. I'd suggest the 'level' method unless you completely rebuilt the forks, cause how much might have been retained in the forks. The 20wt oil is Progressives recommendation for a GL1200.

Progressive install instructions
download/file.php?id=34232
download/file.php?id=34163

[WingAdmin]

ATF, steering fluid and most hydraulic fluid is 10 weight.

15 weight was too viscous for my GL1500 and made the front end far too hard. 10 weight and ATF both work equally as well, and seem like the perfect balance.

[tbeiler]

The forks as well as the Trac system was completely disassembled and cleaned out.
So, I will need to prime the Trac system and then measure from top of fork.
Also see that the tighter wound end of coils should go to the top of the fork.
When I took them out they were installed opposite, the tight wound end was installed to the bottom of the fork.
Not really sure what difference there is one way or the other, but as long as both are the same would make sense to me.
According to the Progressive instructions, they recommend a 20 W oil for the GL1200 and a 30 W oil for the GL1100.
Thanks for all the input.
Thanks.

[golden highway]

What weights of oil did you try, what didn't you like with the other weights and how did it affect your ride?
What made you choose to go back to the ATF?
Are you running progressive rate springs or do you have the standard springs installed?

I tried 10w and 20w fork oils. Overall the ATF gave me the best ride and control. My springs are stock but I am not sure that makes a lot of difference. Someone told me ATF was like a 7 1/2 w but I don’t know how true that is. I used ATF in my CB900c, Gl1200 and my GL1500 and like the results. All my motorcycles had stock suspension. I figure if Honda is recommending ATF it must be be a good choice.

[tbeiler]

So the forks are now assembled with correct level of oil as per Progressive's recommendation.
I used the 20W Maxima Fork Oil. It happened to be locally available.
Trac system primed first, then got the final level of oil minus springs at 140 mm or 5.5" from top of compressed fork.
Forks are now installed on the triple tree.
No leaks noticed after a few hours anywhere on the forks, looks like everything is sealed well.
The front wheel and brake hardware gets installed tomorrow.