I don’t have any official RS 0w30 oil, so can I just use 0w40 engine oil? I know it’s not officially sanctioned, but by common sense?

No

I’m sure its fine, it is just lubricating the lower legs, the only issue might be if there is anything in it that might swell the dust wiper or seals. I used normal 15W oil like you used to use in RS lowers and everything is just fine.

In warranty use OEM stuff

Out of… YEAH engine oil fine, been using it for years in my revs, pikes and rebas. Just make sure it’s fully synthetic. A3b3 or a4b4 spec will be fine.

AS above, once out of warranty, all is OK.

Yep,as long as the weights the same ,oils oil,car motorbike suspension etc etc !

Yep its fine, clean oil is better than old dirty oil! I’ve been doing it for years.

Been running 5w30 Castrol gtx ( fully syntehtic) in all my rs forks for years with no problems

since the first day I use synthethic engine oil for my 2005 pikes,
until today no problems…
these pikes are still in every day use,
just for commuting now…but still perfect 🙂

So why did RS change from the 15W to this new 0w30 oil (bearing in mind that I have no idea what these numbers refer to)?

Engine oil and fork oil are very different in terms of what they are intended to do, Fork oil has anti frothing properties unlike engine oil that is temperature sensitive and built around operating tolerances, also engine oil has some nasty chemicals in that (in theory) would break down rubber and plastic fork internals over time.

I would only use fork oil of the correct weight ( motorcycle oil should be ok )

you cant go wrong with Motul motorbike fork oil !!

in february I did the anual lower leg fluid service on my forks..
all parts on the mentioned pikes where fine, so at least in my case no damage on plastic or rubber..dustseals, stanchions,bushings all fine…
but ! sorry for slightly offtopic..
all my pikes (3) had some metall parts remaining from milling in the oil floating (in the lower leg assembly)…as they where brandnew, direct from the factory..

Gavgas yes you are correct but the damper and lower lube serves two different functions. The damper oil has anti foaming properties yes as the oil needs to pass through tiny holes on compression and rebound.

The lower lube is there for lubrication only.

The rock shock stuff used to be torco fully synthetic motorbike engine oil. I am not sure what they use now to be honest but it won’t be anything special.

A fully synthetic standard grade engine oil will be fine – you are right though, you do need to look out for any additive packs like EP (extreme pressure) as these can be aggressive towards nitrile / viton seals used. Just avoid any oils marked as anything special.