Synthetic oils

The main difference is whether they use a Type 3 or Type 4 base. A Type 3 base synthetic is petrolem formulated, where Type 4 bases are chemically formulated. Mobil I use to be a Type 4 and recently went to a Type 3. Amsoil & Royal Purple still use a Type 4 base whereas Mobil I does not. Tests have showed there is no difference between the two bases other than seal swelling and distortion, but the true synthetics have been crying foul as they are more expensive to produce. Ther is a ton more info on this subject, but I tried to give you the Readers Digest version.

This came up first thing on Google search.

All I run is Amsoil.

There are other comparisons between Amsoil and Royal Purple if you look.....Guess who wins???

http://www.bestsynthetic.com/comptest.shtml

Two sites I highly recommend you check out:
http://www.bobistheoilguy.com/

and this guy is highly recommended, he will analyze your oil and actually work with you to select the best oil for your application:

http://www.dysonanalysis.com/

Here is a link to the lab I use for oil analysis.
http://www.oillab.com/

A few years ago the president of this company came and spoke about Amsoil, he had nothing but good to say about it.

Mobil 1

Just becuase I have used it for awhile had good results and its easily found. And a bit cheaper.

my .02

Differences in the various oils (in addition to Gp III or IV stock) is how much PAE and PAO they have. Also there are differences in the add packs.

Bottom line is that any of them will probably be overkill for most peoples' application.