Rotella synthetic (as well as MANY other oils marketed as "synthetic") is made largely from clsss III oil stock - mostly hydrocracked oil. It is inferior to and is much cheaper than true synthetic, but has some of the more desirable properties of "true" synthetics. It has substantially inferior temperature/viscosity characteristics unless considerable quantities of viscosity improvers (VIs) are added and a much higher low temp pour point (all other things being equal).

Mobil 1 oil is largely class IV (synthesized) oil with minimal VIs while Shell Rotella, and most other major oil company synthetics (Castrol, Pennzoil, Valvoline) marketed in the US are made from mostly class III base stock, but are called "synthetic" because US courts have ruled that such a subterfuge is permissible! Other full synthetics available in the US include Amsoil and Red Line - and there ARE others.

There is NOTHING wrong with using class III oil (or conventional oils either for that matter) IF THEY MEETS YOUR NEEDS, but they should be clearly identified and cost considerably less than oil that consists largely of class IV stock and is much more expensive and behaves much more nicely in VERY cold or very hot temperatures!

Marshall