After flying around for an hour or so to warm it up before draining,
I happened to notice that oil on the dip-stick is 'beading', rather
like raindrops on a window pane.

Am I right in thinking that this oil has wetting additives that have
boiled off in use?

I don't think so. Seems to me that you had water in the oil.
I had it 2 weeks ago - first time. In my case I had the starter adapter
off for service and the engine wrqapoped in plastic.
AQfter my runup I n oticed the problem that you are having and took the
aircraft straight to the shop. They told me water in the oil - and as
the oil was new, go fly 45 minutes and it wouild boil off - which it did.

Tony
--

Tony Roberts
PP-ASEL
VFR OTT
Night
Cessna 172H C-GICE