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Right man as some said ya can jump on here then buy the FLUID [not oil] online!
Viscosity Im not going to get into, check out at an LBS next time ya go there, what they have in there mechanics bay, I'd be surpised if they can work on a DH bike if they don't use Moto cycle fork FLUID [not engine oils or Automatic oils etc]
For Damper side, you may be best with std bike Fluid, for lower legs depending on fork type ya have, I would use a Moto X FLUID. same wieght rec or with rec speccs of ya fork manufacturer.
e.g Motorex would be my number on choice, Motul, or Bel Ray
Moto X Fluid comes in the same FUILD weight as MTb one key thing is the quality of the FLUID, Motorrex for example is one of the best for maintaining its viscosity over time adn during use, e.g like any fulid it breaks or tries to break down under dissipation heat and cooling, and can contaminate, so best is best,.
Like a car put cheap oil in get high engine repair bills, same with fork, put in quality FLUID [specific] not engine oil and you get what you get.
If I don;t service my car myself I buy my oils and take it in and watch them do the work, I only use Mobil in my cars, in my bikes I prefer Motorex for suspension and ATF in my Coke n bourbon, usually while doing maintenance
check ya specc's then order online any of those 3, some may prefer others, Motorex is hard to pass up, BR for you USA boys or Maxima :thumbsup: