I will give you my 2 cents on this issue for what its worth. The motor was engineered with bearing clearances and oil passage clearances, and yes, leak clearances re the oil pump and gaskets, that were based on dino oil. Synthetics are really excellent and hence, modern engines are designed for their use. The modern designs take into account all of the terrific characteristics that make synthetic oils so good at what they do. Now, using a full synthetic in an old 261, even a recently rebuilt 261 to me is just asking for trouble from leaks to bearing cushion.
I know a lot of guys love the Rotella. But that lubricant no longer has the zinc and phosphorus package that our old motors need so I am not totally sold on still using the current formulation of that stuff.
On the other hand, there are a couple of companies selling dino based oil that has sufficient amounts of zinc and phosphorus. I prefer Penn Grade 1 which is Pennsylvania oil (its actually the old Kendal 1 oil) with a synthetic additive package and zinc and phosphorous. I am particularly hard on my motors, and one of my 261 motors sees as much as 5500-6000 rpms without issue. Other companies sell something similar so you might consider skipping a full synthetic oil and use oil that is primarily dino based with the necessary additive packages to keep the motor lubricated, the bearings properly cushioned and the cam and lifters happy happy happy.
Last edited by Dragsix; 02/08/2022 7:06 PM.
