Engine Oil Discussion

I am sorry for bringing this up again but the more I'm reading about it the more I get confused.
I have a high mileage 225 Slant Six which seems to burn and/or loose a bit of oil. I also seems to be never rebuilt. The oil pan is a bit wet but it doesn't seem to be the only issue.
Anyways, I am planning to change the engine oil since the level is low and I don't know what the previous owner had filled in.
In some forums, they said they put in synthetic 10W30 or 10W40 into the Slants while some other say synthetic stuff isn't good, just like what my father said who used to be a mechanic (and ownes some old Volvos) . Some zinc stuff is also required, some people said. My user manual says "no straight mineral oil".
An oil company here in Switzerland makes special oil for old cars. I have found one with extra zinc, based on mineral oil and SAE 50. I now don't know if this will be fine for my Slant... I have read about the Chevron Delo but I would have to import it from the States.


My question now is what oil is not good for the slant, and what do I have to check before I fill in the oil.

My Slant was built in early 1974 with a 3 speed auto, in a 4 door Valiant.