John --

It could be a problem with worn bearings, or possibly sludge buildup in the sump (which can block the pick-up screen).

Or might be a failing oil pressure sender (which is basically a pressure sensitive switch).

When your engine is under load, it tips slightly. The late '70s to mid/late '80s 240s were famous for crumbling engine harnesses. Furthermore, the wire to the oil sender is right behind the alternator and routinely gets oil soaked during oil changes, and gets baked from the exhaust pipe heat. This causes even worse degradation of the insulation.

If the wire is intermittently touching ground as the engine tips, it can cause the light to flicker.

Your mechanic should remove this switch and install a genuine (mechanical) oil pressure gauge on a hose. You and he could watch the true oil pressure under varying engine conditions, and this would narrow his focus while solving the problem.

I'd first eyeball the wires behind the alternator—it's free, easy, and quick.

Don Foster (near Cape Cod, MA)