If Mobil is working for you, and you have a dependable source for it, I wouldn't pay 1/3 more for Pennzoil. This is from somebody that hasn't used little except Pennzoil for most of the last 30 years. Even if jumping around from one thing to another doesn't cause problems, if you do have problems, it is harder to figure out why. I continue to pay a growing premium for Pennzoil, because I know I will always be able to find it somewhere.
Price is more a marketing matter than quality. Look at Fram and ST.