Sure it is engine oil and not transmission oil? The differential is sealed by rubber on the driveshafts and if those are original, they might have started cracking and oil might travel across the subframe.
Replaced mine years ago and first tough it was the engine.
As for engine, filler neck or the dipstick tube loose on the block? Or just traveling down from the rocker covers?
Otherwise, clean and see where oil or other fluids starts appearing?