I had the exact same problem on my 855T5A (72,000km then), and mine took a while to figure out.

Some time after an oil n filter change, i began noticing little pools of oil appeared under the car overnite, later i narrowed the occurence down to it "leaking" only after a hard drive. Much later, after a hard(er) drive on the highway i noticed engine oil all over the top of the engine. but on all occassions i checked the oil level to make sure it was sufficient, and it was.

after a while i began to suspect overfill, with the oil spurting out of the dipstick funnel. if i recall right, the dipstick was showing some half cm above the max mark. next service, i replaced the engine oil and ENSURED the mechanic filled only to the halfway mark. (them mechanics seem to always think the oil level will go down a lot once the engine's started and they "compensate" by overfilling) no more problem ever since.

hopefully this is the cause of your oil leak cos the solution would be so simple.

rgds

jeff

95 855T5A, SAM chip, IPD swaybars, full S/sprint exos, Bilsteins