Well, I've been a bad boy and haven't paid much attention to state of the art, but I do know that they continue to change the oil formulation and what is good for my 2020 RAV4 would probably cause my 65 to throw a rod in 20 miles. I've not changed the oil in the 65 for several years (it gets very few miles), but its time.
I'm sure its been discussed, but I could not find it with search. Should I:
The old Ford i6's had very light valve springs and could run on any oil under the sun. These engines are NOT hard on oil and do not have extraordinary AW requirements.
-- run a diesel engine oil?
If you can get it cheap? Sure! It's certainly not going to hurt.
-- buy conventional high mileage oil and add some ZDDP additive?
Definitely not necessary, you do NOT need added ZDDP, an HM oil would be fine by itself and might help with your leak.
-- spend piles of money on Lucas or Amsoil oils specifically marketed for really old engines?
Avoid Lucas like the plague. It's like mixing dog faeces in with your steak sauce.
While AMSOIL makes some excellent lubricants, I doubt you are going to see a benefit from them in this application.
The engine leaks a little oil from the pan gasket and a little from the rear mail I think. By little, I mean 10 drops over the winter. But I would not want to try a synthetic or probably even a semisynthetic, unless one of you can argue very persuasively.
Thanks in advance, from a long time lurker.....
Then the HM oil (by itself, not defiled with additives) would likely be your best bet.