I'm having a leak on the back of the passenger side of my engine.I replaced valve cover gaskets but still no change.What else could it be ?Since you are new to turbo Buicks, I suggest you very closely examine your engine to determine the it is actually the rear main seal leaking?
More times than not, the oil leaking at the rear of the engine is NOT the rear main seal.
Over the years, I have had hundreds of customers complain about oil leaks and say it is the rear main, but only two or 3 have actually leaked there.
If you are not a mechanic, you should go to a shop that is knowledgable and honest that can examine you car on a lift to see if oil is present around the front edge of the flexplate starting at the crank.
From that point of view, the rear of the valve covers, oil pan and cooler adapter can also be examined as these are common places for oil leaks to occur.