I used to work as a quality control technician at a major hydraulics firm and had acess to over 1 million dollars worth of oil testing equipment.
I decided to test a number of automotive oil brands for personal enrichment. Upside to the tests, synthetic is VERY good! Downside, some domestic brands are almost as bad as no oil at all!
Tests included: particulate contamination (how clean the oil was - huge factor in longivity) - Viscosity (is it really 10w30 and how does it change with temps) - Recirculating ball test (run a ball bearing against a shaft at 3000 rpm with an oil drip - measure at intervals for shaft wear).
In particular, both Pennziol and Quaker State oils were virtually worthless. Both fouled my Royco particle tester and both failed miserably in the recirculating ball test. (They cut the shaft in half after only several hours of running!)
Best domestics were Valvoline and Havoline (non-synthetic). The Valvoline in particular was almost as good as a synthetic on the recirculating ball test and was easly the best domestic natural oil.
Synthetics tested included Mobil 1 and Amzoil. Amzoil was better than Mobil 1, but both were better than any other oil tested (not a complete test, some new oils have hit the market since of which I have no info). The Amzoil did not even leave a mark on the shaft, which indicated to me their break in instructions were correct - ie, run standard oil for around 10,000 miles before changing over to synthetics. Valvoline left a shiny spot on the shaft, but could not be measured after a week of running. Other domestics wore groves in the shaft after days of runnig.
In doing mechanic work for 20+ years on the side, I have also noticed that the motors using Pennziol are usually full of crud, and I recently had to replace the crankshaft on a Dodge Dakota with less than 80,000 miles. Two bearings were completely gone, Owner swore by Pennzoil. Took the block to a local machinest to bore it and he told us the type of oil used in the motor. He said he sees it all the time.
In another incident, I started running Amsoil in a Chevette with 22,000 miles. I followed their standard 25,000 mile change with a filter change at 12,000 (add 1 qt to fill to caoacity). After 220,000 miles of this the car only used the one quart of oil per 25,000 miles from the change. I sold the car and it is still running. I don't know the current mileage.
One further story... I ran an F150 with a built 390 and 38's and had difficulty moving it off the spot on some of those -20 degree mornings in Wisconsin. I swtiched all the diffs, trans, and transfer case to synthetic and was able to drive away normally no matter what the temps.
All of this is anecdotal evidence, but I know where my money goes! I use synthetics all the way!
