Dave, I'll start with the fact I've never heard of Kendall's blend numbers before either. Lets start with facts. Petroleum base stocks are Groups I, II & III. Synthetic base stocks are groups IV (PAOs or Polyalphaolefins)and V (Esters/ polyesters).
Additionally, the group III oils have been classified in the USA as a "synthetic". That would make some premium petroleum oils as being qualified as synthetic many years ago as they are now in the USA. "Synthetic Blend" is a marketer's dream come true.
A 60/40 blend is a very optimistic percentage. If a premium petroleum oil is Group II or III base stock, that is fine. If a blend is a mix Group II and III, it may well be a very good oil. If it is a blend of Group III and Group IV, I would call it a genuine "synthetic blend". Percentages are most likely to be 90-95% petroleum and 5-10% synthetic. A synthetic blend of less than 5% actual synthetic would be no surprise.
You can cost out what expense best suits your budget. Whatever blend you choose, keep the OCI under 7,500 miles. Another option which others have already suggested is to add on quart one genuine synthetic to four quarts of premium petroleum oil. You can count on AMSOIL or Royal Purple as being real synthetic Group IV oils. At $10 per quart, they should be genuine. Then you have some certainty as to the actual "synthetic-blend" percentages.
If you really want to prove the make-up of a synthetic blend, Oil Analysis is the only way to really know.
Dr. Olds
1994 Intrepid ES 3.5 V6 AMSOIL ATM 10w30 Signature Series. 206k miles, no issues.
1969 Olds Toronado Rotella 15w40 (NOS) (Yes, I have a stash of excellent old Rotella & CI rated Castrol GTX 15w40).
