Good point!
You can expect a lot of mileage out of a vehicle using mineral based oils provided it is not abused and has gotten regular maintenance. Many like you have done it and are well into the many 100k's on the clock.
That being said, almost any synthetic oil, be it Mobil or Amsoil or others, can be expected to be pretty darn good and a definite improvement over regular oil.
Again, I mainly buy Mobil 1 because it is well developed, readily available and is cheaper than most brands on the shelf.
