3800 Series II Oil Leak

I have a 98 Bonneville SLE with a 3800 series II engine, the engine has about 110,000 miles on it and it runs great. Last year at about this time I had to have the upper intake replaced for failure, which was not a big deal because I was just reimburssed the entire amount for labor and parts from GM due to a recall. Now, this year I have a cronic oil leak which I never had before. The oil is coming from somewhere around the underside where the small platic cover is on the bottom of the flywheel between the oil pan and the tranny, but I cannot see exactly where it is coming from. I dropped the oil pan and replaced the oil pan gasket hoping that I would get luck, but I did not...go figure. I was wondering if anyone has had or knows of this problem on the 3800 series II engines. According to a mechanic that I talked to today he told me that the upper main seal is prone to failure. He said he would drop the tranny, replace the seal, and fix the leak for $375. Has anyone ever replaced theirs, did you do it yourself, & how involved is it? Did it still leak after repair was complete?
Any help would be much appreciated...

I would have to agree with the mechanic about your problem. The rear main seal is going bad and it needs to be replaced. $375 is a fair price to do this. I have read a lot of posts on this forum over the last several months and have not seen but one or two that address this problem. So I would say it is not common. One question, did you recently change to synthetic oil?

No synthetic oil here....I cannot see where it is coming from when underneath, its just a small leak, I think I might have found a missing bolt.
If you stand on the driver side, lean over the fender and look down just underneath what I think is some type of cylinder shaped sylinoid, just to the far side where the front exhaust pipe wraps up over the tranny, there is a bolt, looks like it bolts the bell housing to the block, it is at about 12:00....just to the left there is one just like it as well as to the right, I am missing the one on the right. I stuck a piece of wire in the bolt hole to check the depth and it was about 1-1/2" deep, cant tell if there are any threads in it though. It is a 18mm bolt I believe...Can someone tell me if there's has a bolt in this place. It is a 3800 series II. I think this is my lerak because there is some light fresh oil on the surface and while I had the car runing it appears that there are small air bubbles at the seam. This area is also almost directly above that little plastic flywheel cover.