No need to find out what brand they used prior, just buy whatever you feel is the best your money can buy. As far as switching sythetic and dino - you can only switch from dino to sythetic safely. You can not switch from synthetic to dino. The synthetic obviously is made up of a different base which gets absorbed into the gaskets and swells them up. This is one of the reasons why some people can see a performance gain on an older engine, they get better compression due to the swollen gaskets. If the engine has previously had synthetic oil and you switch back to dino oil, the gaskets CAN and most likely will shrink and cause leaks. Now this is not a 100% guarantee, there may be some people who have switched from synthetic to dino with no problems. Depends on how tight your engine is, but there is a very good chance that you will get a leak.

The shop that I go to for my oil changes uses full synthetic Quaker State oil and a Quaker State filter. It might not be as good as Mobile1 but at $40 for a full synthetic change and a free car wash, I can't complain. And I figure the Quaker State full sythetic has got to protect better than any dino oil.