hi robert s.

did i see you in a video wearing a steelers hat?

one thing that seems sure, don't run conventional oil in 5 cyl volvos
(or any car for that matter). It clogs up egr plumbing causing plenty of trouble.
also my volvo trained indy reports that conventional oil also has a nasty habit
of clogging up oil galleries in 5-cylinder heads with that same hard crusty stuff
you find clogging up the egr port to the intake manifold.
clogged oil galleries in the cylinder head are a nightmare likely promoting
excessive valve wear and failed head gaskets.

i looked up "castro" gtx oil and see its conventional oil
and there is also castrol (usa div of castro oil, havana) gtx and gtx synthetic.
maybe there's a gtx synthetic blend? easy to grab the wrong one.
unless you change your own oil, all bets are off as to what you got.
i think the last indy that charged the p/o of my car for an oil change was a robber.
the oil at very low mileage and time was as black as putin's heart.
there's another good reason to change your own oil.

the photos of castrol gtx and spec sheets at lowe's are rubbish, where they
hide the api/--? spec altogether.
it would be nice to know the breakdown temp for oils and have it printed on
the container.

i have some jugs of mobil-1 and castrol edge full synthetic 5-40 api/sn euro blend here
5w-40 api/sn euro blend is what mobil-1 says for my high mileage redblock 940.
mobil says 10,000 miles or a year for oil changes.
i'll be changing my oil sooner since the p/o and volvo shops were running
conventional oil in this 940. ugh! you can see the synthetic oil cleaning out the engine gunk.

cheers, bill