Ok, so what's the difference between a 5w specific 'Fork Oil' and a 5W/40 fully synthetic engine oil? If it's good enough for a Porsche engine… etc…

ones for damping and ones for lubrication?

I dunno…

my family say i often talk about fork oil !! 🙄

I think 5w40 means it reaches a 5 viscosity rating at 100 deg C using the winter grade viscosity. The 40 means the SAE standard viscosity also rated at 100 deg c. Either way you'll have to be seriously hammering the trails to acheive anything comparable to 5 weight fork oil!

More info here:-

Oil

yeah fork oil and engine oil for different applications and different viscocities. I use motorcycle fork oil as its cheaper than "mountain bike specific" stuff and havent had any problems in 10 years of doing so (as its no doubt the same stuff…).

its no doubt the same stuff..

Best to avoid brands/types of motorcycle shock oil that contains "seal sweller".

Loads of info here

http://www.peterverdonedesigns.com/bikesuspension.htm