My personal preference is Chevron Delo 400 15W40. The Shell Rotella line is also a good choice. Both of these have the added zinc our older engines need.

Here's my M115. I had not adjusted the valves in five or six years and I figured it was time. This is how the valve cover and train looked.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now