Interesting theory, but I have a hard time believing that the oil warms up
appreciably in 60 seconds.

My thoughts:

The application of full power on an aircooled engine causes things to
heat up very rapidly since there is little thermal inertia. On startup
& taxi, the oil bypasses at the relief valve & doesn't get much access
to the heat of the engine until you get to a certain point, where it
really begins to remove heat, quits bypassing and then warms up
quickly. The specific heat of oil isn't very high yet the heat input
once things get to spraying around the crankcase are probably pretty
substantial.

Somewhere the intake manifold might be changing from a mixed
liquid/vapor flow to an all vapor flow at a corner or an obstruction.
Look up the manifold and on the carb gasket when you do the throttle
plate bit. Check for loose clamps etc on the manifold tubes - but it
sounds like your mechanic etc (I. e. you) are pretty well on top of
things and have done that already.

Q - How well did the Lycoming clearance measurement thing work? I
built one but have not used it yet. Was your's per their plans?