I had similar issue with my M52 engine (oil on electrode side of spark plugs) on my 1997 E39 standing at 215K miles on the odometer. It turned out to be the valve stem seals, which I replaced. They were hard, brittle and wider than the valve stems. Do you also get a puff of blue smoke when starting the car only after it has sat for a while, e.g. in the mornings? That would be a definite give-away your issue is similar to the one I fixed.

__________________
'05 E53 X5 4.4i, '97 E39 528, '07 E92 335i, '16 F86 X6M.