What weight fork oil

[95bravo]

'76 RD400 with stock forks.  The parts manual in the Tech section of this forum says 10, 20 or 30 weight oil in the forks.  I see fork oil on the shelf for different weights (5, 7 and 10).  What weight fork oil do you use?   (This is NOT meant to devolve into a debate about what type of engine oil is best) ;)
Thanks in advance. 

[1976RD400C]

You may have to experiment some. I think the RDs came with a fairly light oil. 30wt seems a bit much. I would try the 10 or 20 and head down a choppy road. If you notice the front wheel is not staying firmly on the road try going to a lighter oil.

[teazer]

Old bikes like RDs used thicker oil than modern bikes because of the design and manufacturing tolerances.  Modern bikes use really light oils and that's what dealers stock.

For an RD, I'd start with 20wt fork oil and try pushing down on the front end of the bike with a mate holding the rear to stop it falling over (that can be embarrassing).  If the front end goes down in less than say 1 second and springs right back up, you will need to go to 30wt.

With stiffer than stock springs, the damping oil needs to be more viscous to work with the stiffer springs.

[Dvsrd]

Racetech recommends 15wt fork oil, when using cartridge emulators.
However, the rebound damping is unchanged after converting the compression damping circuit using emulators.  And the bulk of the damping is rebound anyway.
So I would go with 15 wt oil.

[Gil Gallad]

i've used atf in my front forks, all 70's bikes and it works well [for me]. used nothing else for many years, but there again, mine are all suzukis and i'm only 12 stone wet through - don't ask me what that is in pounds  ;D
cheers, gil.

[85RZwade]

I don't know what that is in pounds, either, but I've always been told that atf was 7.5 weight.
Wade

[Kawtriplefreak]

Plus 1 for ATF. I use it in all my bikes too.

[NoRiders]

I've gone for Valvoline 15W synthetic fork oil on the NSR125 JC22 forks....mainly because the bike can wear the Valvoline decal as the colours suit the overall intended paint job :)
Not a scientific choice but I did go for the heavier 15w grade.....fork oil is fork oil ain't it?

[motodreams]

Ya, going to be huge differences between these options in the suspension feel.  Run factory recommendation plus a grade heavier.  If they recommend 5wt and you use a 30wt motor oil there are going to be some major differences.  Recommend grabbing some of the generic branded oils vs. the bike dealer stuff.  Think the price was 3x @ dealer.  ATF is essentially the same price so not sure why you would run that over the correct fork oil.  Would say this is one area where the 'proper' weight will matter more than anywhere else (other than maybe using premix oil in injector).