If you need to add in between oil changes, go with what you have in your engine now. If your last oil change was with Mobil, go with Mobil now. As for the weight, go with the recommended (on your oil filler cap or in your Owners Manual). My '95 uses Valvoline 10W-30 (10W-30 being recommended for my vehicle). If you don't know what is in your engine now, one quart of Valvoline (or your prefered brand) won't hurt for the next 1,000 miles, but if it were me, I'd just get an oil change now--and be consistant on the brand/weight for as long as you own the Bonneville.
I would also recommend that you do a search on this forum for "oil" and read all the posts. There are many opinions on preferred brands, read what others have to say and select a brand based on others experiences--good or bad. My preference is either Havoline or Valvoline for my cars. I use Valvoline in my Bonneville and Havoline in my Lincoln Mark III. I am satisfied with both.
Retired Moderator, The Bonneville Club www.bonnevilleclub.com
RIP Oldsmobile, April 29
1995 Bonneville SE Supercharged (Stock)
Current Mods: Dirty GYM Sock air freshener.