Here'* my personal recommendation.
Some cars like the Bonneville GXP benefit from being able to use fully synthetic oil and get a higher mileage per oil change. Mine for example will get 6-8k miles on one change. However, on the Regal'* 3800, I never got over 4000 miles before I could feel the engine start running rougher and had to change the oil. Its simply not worth it when you have to change it every 3k anyway.
My recommendation is to get 4 quarts of conventional oil and a bottle of rislone engine treatment. Pour the rislone bottle in first, then add 3 quarts of the conventional oil, and use the 4th quart to top it off.
More important than the actual oil is the filter, and this is something most people ignore. The filter you use is EXTREMELY important. Please take the time to read these two articles:
I have heard that our oil filters actually get clogged up before our oil change is due. For this reason, I decided to go with a larger filter. The factory filter for our cars is a GM PF47. What you may not know is the PF52, a filter that'* 1.5x longer and costs exactly the same, also fits on our cars. There is absolutely no disadvantage to doing this, and you get better filtration.
When you go to the store to pick up a filter, ask for a filter from a 1992 Chevy Lumina Z34 (3.4L) as those are one of the cars that use the PF52. I believe other 4.3L Vortec motors also use the PF52. Just make sure you have enough clearance and you're good to go.
2005 Pontiac Bonneville GXP - For Sale ----- 1995 Buick Regal GS(X) - L67 madness
2000 Buick Regal GSE - Stock...for now
----- 1987 Jaguar XJ-* V12 - 12>8