I had a previousl experience with an oil leak and through some research here is what I found. My Volvo (95 850 Wagon) had a blockage in the breather system. The breather system functions similar to a PCV valve in my MG midget in that it vents excess crankcase pressure. However, the blockage eventually caused the rear-seal to rupture (due to the pressure finding the path of least resistance out of the engine) and this caused a significant oil leak.
There was no indicator light, nor diagnositc code which pointed to the problem, only an oil leak. Now, the pressure could have blown out any of the other seals, however as luck would have it, this was the most expensive one to rupture. So, it cost a bit of money to get it fixed, but now it's as good as new.
Point being that the oil leak on my Volvo was a result of crankcase pressure. This is apparently not uncommon.
Important note though; when the shop replaced the rear seal I ensured that they also took care of theperceived culprit...the blocked breather system.
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