TallPaul,
Best way I know is to look at the pour points and borderline pumping temps. If the PP of a 5w-30 is below -35F (below -30F for a 10w-30), chances are it's a Group II basestock. Noack volatility of the Group II stocks is also going to be lower,and they will have a Viscosity Index that is typically 10-15 points higher.
As a practical matter, the GF-3 and CI-4, diesel upgrades severely limited the use of Group I, solvent refined petroleum basestocks.
Group II's include:
Chevron Supreme
Pennzoil "purebase"
Petro Canada - some of these are actually Gp II/Gp III blends for an excellent price.
Castrol GTX
All the SAE 5w-20 oils - most of which also contain some Gp III stocks to help them pass the double length Sequence IIIF test.
I think the Chevron Supreme and Delo 400, 15w-40 are the best values out there, bar none. The Supreme not only has an excellent Group II stock, but a very nice additive package as well (borate ester and MoDTC). Delo 400 consistently show excellent oil analysis results and very low rates of oxidation.
I'd run the Supreme 5w-30 in the winter and the Delo 400, 15w-40 in the summer, if I was using a petroleum oil in a high performance, gas engine like my 1.8L, 225 Hp,Audi TT turbo ....
Tooslick
www.lubedealer.com/Dixie_Synthetics