I have a forklift with a commercial slant 6

Of course the above would not apply to an engine with hydraulic lifters (your self adjust thought) but I think most slants are adjustable aren't they.
Maybe I'm wrong but you mentioned the scenario of it and I came up with something that might apply that has apparently not been checked, when in fact it should have been probably long before this point in the game.
I'm actually kind of surprised the seasoned guys haven't checked that yet.

Motors don't quit after 10 min for no solvable reason.:D