My Pikes have run untouched for a while, but little used & lubed the stanchions after each ride. All was well until I decided to check the pressures – top +ve pressure was fine but when I attached the pump to the lower -ve valve it pushed oil into the pump. A bit of further investigation showed quite a bit of opaque oil in the valve.

Simple questions – should this happen? If not, what's wrong? And are there any really, really basic simple-to-follow instructions on how to service them?

Thanks

should this happen?

Yes.

If you don't want it to do it, turn the bike/fork upside-down before setting -ve air pressure

logic told me that, but being a numpty I wasn't sure. Also, just found a couple of Youtube clips where oil doesn't come out.

Anyway, if all's OK I'll forget about it!

You can get oil out of the positive chamber as well sometimes.

wouldn't hurt to top up the oil in the air chambers at some point, should be fine for the time being though

This is normal. Careful you don't get oil on your disc!