The compressor is made by 'Valley', buts its just a cheap, rebadged
generic compressor made in China..

Its an upright, 3hp, single piston type with a very similar 'engine' to
my old Oil Free 24L version (again, a cheap, rebadged import).

I think that I will drain the old oil out and put in some new stuff...
I found in the (limited) instructions that it suggest N32 oil in the
winter and N68 in the summer.. I guess this "ISO" number.

One questions about oil changes is, how do I get all of the old oil
out? Obviously I heat it up, open the drain plug and tip it out, but do
I have to get EVERY drop out? Does it matter if there is still a little
bit left over?

I think I know where I can get the right oil in the UK, so am going to
change whats in there, just in case.

All the best

Jon