Decided to service the lowers on my Talas fork last night; hadn’t done it before but thought the lower fork oil wouldn’t be that big a deal as it’s just a squirt to keep stuff lubricated right? Wrong, turns out the damper side needs a whopping 165 mL of 10 wt. Didn’t have any, so threw in 80 mL of 5 wt and 85 mL of 15 wt.
Both the same brand generic motorbike fork oil. Took the bike out today on a big ride and it seemed fine. Is it OK to mix oils like this? Or would you drain it and refill with 10 wt?

Yes it’s fine. You just get a mix that has a specific weight that is somewhere between the two original oil weights.

Mixing different weight oils is commonly done to tune suspension. For example, equal amounts of 10 and 15 weight oil to make 12.5 weight.

Is there not some funky and unintuitive relationship between oil weights, so you don’t get an exact midpoint by mixing? (leaving aside that any 2 10Ws are likely to be different anyway)

Erm technically im pretty sure it would actually be a 56/44 mix

Yes there is NW, but JoeG’s method it’s close enough. Shell (as in Oils) have a free calculator called ShellMix that’ll calculate the exact mix.

Yup, can’t use weights as they are pretty meaningless. You need to use centistokes (cSt).

http://www.peterverdone.com/wiki/index.php?title=Suspension_Fluid

http://www.widman.biz/English/Calculators/Mixtures.html

As above really.
Yes you can.
“wt” designation is meaningless, cSt are what count.
When I used to do this with silkolene, I think 2/3 of 5 plus 1/3 of 10 made 7.5

Bottom line though, if you’re happy with the compression damping and can adjust the rebound to where you want it, you have the correct oil “wt” irrespective of any numbers.

As you have damping adjustments even 100% wrong weight might just mean you close or open the damping a bit to get back roughly to where you were. Plus wrong weight clean oil must be better that the right dirty oil, which has probably changed viscosity anyway due water and suspended solids contamination.

As you have damping adjustments even 100% wrong weight might just mean you close or open the damping a bit to get back roughly to where you were. Plus wrong weight clean oil must be better that the right dirty oil, which has probably changed viscosity anyway due water and suspended solids contamination.

As you have damping adjustments even 100% wrong weight might just mean you close or open the damping a bit to get back roughly to where you were. Plus wrong weight clean oil must be better that the right dirty oil, which has probably changed viscosity anyway due water and suspended solids contamination.

If the oil is the same brand and both are fully or semi synthetic, this should be an issue (although the semi synth won’t feel as good)

Refer to above links for mixing guidelines too as it’s not 50/50 to make it the ‘tweener’ wt 😉