jenko
+1y
My rule of thumb is old oil for old engines.
The tolerances/clearances of old engines don't match up well with the flash fully synthetic engine oils.
One easy to identify point is they tend to burn plenty off as their rings are older (from wear and just being older production techniques) and don't scrape as well as they should.
Shell Rimula 15-40 mineral oil for my B2500 TD.
The new oils have more additives in them for breaking down deposits etc and I'm told you can extend your service intervals with it.
From 5k to 10k I was told by the last mechanic to service it (got a voucher for free labour and thought I'd save the missus the ignominy of changing her own oil, "You brought it, You service it!" philosophy) I have a little trouble swallowing that, I'll push it out to 7k while keeping an eye on it.
Weights depend on your area and time of year.
Hell there is probably a stamp on/around your engine or on your sill which tells you suggested oil weights
Good luck!