You won't get any negative comments about Pennzoil from me- it is, and has always been, an excellent product. Both it and Motorcraft are fully formulated and either will do fine. Or, you could consider reaching for $1.48/qt. TropArtic at Walmart in your favorite viscosity flavor. Both TropArtic and Motorcraft are made by ConocoPhillips, and both are synthetic blends (50%+ Group III base stocks). The good news is that even with post-Katrina price increases, there are still some excellent bargains in motor oils. Don't forget Havoline, either.
Now, about "Group" #s - if you're asking what the difference between Group I, Group II, and Group III base stocks is, here's the short and dirty answer:
Group I is a bottom feeder base stock - solvent dewaxed technology that goes back at least 30 years. Though "dewaxed" by cold filtering after dilution with a light solvent, there's still wax content that has to be controlled with pour point depressants for cold weather applications. (The light solvent is later driven off under the application of heat and condensed back to a liquid for re-use by the refiner.) There's also more aromatic content than would be desirable. Aromatics are great solvents (good for keeping seals pliable), but terrible lubricants. They also have a nasty habit of forming partially oxidized particulates that are prone to forming sludge if not stabilized by detergents and dispersants. Group II is catalytic conversion of undesirable waxy side chains and aromatics to desirable lube molecules under high pressure in the presence of pure hydrogen gas and very high heat - "isomerization", a form of synthesis. "Polymerization" is the building of complex molecules from simple ones. "Isomerization" is the tearing off of undesirable side chains and/or the breaking of undesirable ringed structures to improve an existing molecular structure. In both cases the goal is to saturate the primary carbon chain with hydrogen, only. ("Saturation" - good for motor oil, bad for arteries...) Group III is much the same process as Group II, but left in the pressure cooker at even higher temperatures and over more time to extract an even purer, higher quality base stock - extremely low concentrations of aromatics and wax. Only polymerized PAOs (Group IV) are better - and only marginally at that. Finished motor oils using Group III and Group IV base stocks get around the seal shrinkage and pliability issues by incorporating certain esters - good lubes in themselves and promote controlled seal swelling. The biggest material beef against Group III-based full-synthetic formulations is that too often they're priced as high as Group IV-based full-synthetics. Walmart SuperTech Full Synthetic (Group III) is one notable exception at $3.00+/qt.
