[Closed] rockshox monarch service

Hopefully an easy one - I'm servicing a monarch shock and in the closing steps the service instructions say to add 0.3ml of maxima maxim4 extra 15w50 oil. Apart from the obvious issues with measuring that small a volume, am I safe to use "any" suspension lube oil, say float fluid, or similar?
Google seems to suggest is engine oil, I know I've a pricey bottle supplied by the garage at my last service, though I'm unsure what it is without checking. 15w40 seems easy and inexpensive enough to get hold of but I've no idea what the difference is/what the numbers mean.

Thanks