5W. A good oil that is inexpensive is Motorex. Don't use Belray unless it is the HPI "Racing". I think the best is the Ohlins R&T 43. That is what we use in the shop. I don't trust Motul syn as I got a set of Honda forks once that had that in them and the seals were like 2x the norrmal size
I am quite certain Showa forks on Duc's are factory spec'd w/5w fluid. KTM's arrive from factory w/Motorex in their WP units. They claim it's 5w however when I have changed it, I find the fluid relatively thick (high viscosity) compared to most 5w suspension fluids I am familiar with.
The Ohlins fluid I am sure is fine but the price is ridiculous for a simple mineral based fluid. I know of at least 1 locally based AMA team using Ohlins Superbike forks and they use Redline suspension fluid instead of the spec'd Ohlins fluid. I have used Redline "Lightweight" fluid which they claim is 5w equivalent in WP, Showa and Ohlins forks as well as Penske shocks with superb results. I have spoken with a Penske engineer who stated their units are all delivered with Silkolene 5W. Silkolene and Redline fluids are basically identical, other than price and availability which is largely dependant on location. In SF BayArea, Redline is much lower price and readily available. I know in Atlanta, Redline is pricier than Silkolene and probably harder to find.
The Redline fluid seems to suffer far less oxidation between service interval, witnessed by fluid remaining a pale blue. I've never noticed excessive fade and seals never seem swollen. A friend witnessed some o-ring swelling using Yamalube in a WP fork, chosen because it's what he had laying about. Take care.
The S.A.E. "W" system was made up for motor oil. It really is not a reliable way to measure the viscosity as relative to working in a hydraulic system (like our forks). Using a Centistokes measurement is a much more reliable way to compare. Temperature stability, anti foaming and lubrication are just some of the critical properties.
Yup, you're right, Rick. One should check the actual viscosity rather than the nominal W rating. Problem is....the Showa SS-8 is marked 10W on the bottle, like the SS-7 is marked a 5W....but they don't have the centistoke values/specs listed.
I can't seem to find ANYTHING on the net related to the actual centistoke values for these oils. Found a cross-reference with just about everything EXCEPT the SS-8 and SS-7 (typical, huh!).
Do you happen to know the cSts for these 2 oils? I'm kinda curious what other make/viscosity oils are a close cross-reference to the SS-8 10W oil....