I started seeing oil collect on the top left of the engine. After A LOT of investigation, tests and gasket replacements I found the dipstick not sealing well.

The dipstick had TWO rubber washers. One appears built into the stick, the other may be an add on from a PO?? After removing the extra rubber washer the leak appears to have stopped.

Why did it take 4+ years for this to happen? Recent work included replacing lost cooling tins and sealing the engine compartment from hot air leaks. Maybe that work lowered the air pressure in the engine compartment enough to cause the chaos.

Thanks to 914rubber for the extra thick oil filler gasket.

And if you ever need a rubber grommet that seals around wires and cables, search ebay for "rubber grommet blank" or "rubber grommet closed" - oddly they are sold only in UK.

My dipstick also leaks under hard use....like DE events with 30 minutes of full throttle 90% of the time. Vents are all clear and open, but the crankcase pressure can still cause the oil to climb up the dipstick tube & it doesn't take much pressure to get it to weep out. First I tried diapering it with paper towels, but then moved to clamp on a blanked rubber hose--that fixed it!

Did you close off the crankcase vent at the oil filler?