[George Bongert]

Greetings bigblockv6!

I did the research you suggested, and indeed, my 99 Bonneville (3.8 V-6) does have a roller cam as you said. I also visited the official Mobil 1 Website, and came up with some interesting information regarding Mobil 1 Synthetic Oil and flat tappet camshafts.

Quote from the website:
Question:

"Does Mobil 1 contain the Additive for Flat Tappet Cams?"

Mobil 1's Answer:

For older, flat tappet engines where wear may be more of a concern, we offer a number of synthetic oils which are higher in phosphorous than API SN/ILSAC GF-5 oils. These include Mobil 1 15W-50 (1,200 ppm), Mobil 1 0W-40 (1,000 ppm), and High Mileage 5W-30, 10W-30 and 10W-40 (900 ppm). For all newer engines and flat tappet engines in normal service, API SN/ISLAC GF-5 oils are preferred for better fuel economy and for the protection of catalytic systems with lower phosphorous (800ppm)."

This is a direct verbatim quote from Mobil 1's website which was updated in April of 2012.

My interpretation of this is that there is no reason not to use (or fear using) Mobil 1 15W-50 Synthetic Oil in a flat tappet engine where cam wear may be a concern. As I stated previously, I personally would have no reservations using Mobil 1 15W-50 Synthetic Oil in any engine that I would personally own, whether it would be equipped with a flat tappet cam, or a roller cam. Thanks for urging me to do a little research. I got a little education on both the GM 3.8 V-6, and the subject of Synthetic Oil vs ordinary engine oil. I have bookmarked Mobil 1's website for future reference since it addresses many more engine and lubrication issues than just those of flat tappet camshaft engines.