Just thought I should share this, I had a similar problem in 1976, as soon as the engine got warm the oil light would flicker around the 2500RPM range and would go out when rotating above this speed, it did progressively start coming on earlier and going out later after a few months. Turned out to be broken crank, cracked in the middle of central main bearing journal right where the oil way is. The engine ran fine for about 6 months under these flickering light conditions without displaying any other signs that would have suggested the catastrophic failure that eventually occurred at 60MPH when it eventually let go. I can only assume the reason the crank rotated correctly while cracked for so long was due to the profile of the surface through the crack , almost true like it was cut on a lathe around on the surface, but very jaggered below the surface, and again, can only assume all thrust bearings were within their specified tolerances, which prevented the crank halves from separating for such a length of time. Needless to say, there was nothing that was salvageable.
