One wonders if you can even buy a 30W or 10W30W today like they specified 40 years ago.
Here is a fabulous web site, and fabulous tool in evaluating oil. Look toward the end of the report for the charts, very revealing!
http://www.trustmymechanic.com/motor-oil-bible.pdf#search=%22%2Boil%20%2Bbible%22
One thing is for sure..............using the WEIGHT of the oil as a measure of performance is a waste of time. It means little.
Look at the wide variation in performance you get within a given "weight" category..........this is why reading the info on the can is of little use.
Observations:
Straight
30W oils range widely in performance (best overall ranking is 735, higher is better)
Pour Point: ranges from -6 to -50 degrees, within the same 30W designation. (Hardly consistant)
VI: 95 to 180 within the same 30W designation. (Hardly consistant)
Flash point: 410 to 514 degrees F within the same 30W designation. (Hardly consistant)
So one wonders, just what does the 30W mean when you see it on the can? ANSWER: "not much".
10W30 oils also range widely in performance (best overall ranking is 762, higher is better)
Pour Point: -22 to -60 degrees, within the same 10W30 designation. (Hardly consistant, and why so much variation with a 10W30 rating??)
VI: 129 to 176 within the same 10W30 designation. (Hardly consistant)
Flash point: 400 to 518 degrees F within the same 10W30 designation. (Hardly consistant)
So one wonders, just what does the 10W30 mean when you see it on the can? ANSWER: "not much".
~~~~~~~~~~~~~~~~~~Most of the top scores within each weight category are synthetics~~~~~~~~~~~~~~~~~~~
Now, here are three specific oils that have been used for comparison here before, and all are considered to be good oil. The two synthetics are clearly better performing than the non synthetic, but all would be good.
Mobil 1 15W50 (overall ranking 721)(783 is the highest ranking in the 15W50 range)
Pour Point: -49
VI: 153
Flash point: 446
Castrol 20W50 GTX (overall ranking 636)(786 is the highest ranking in the 20W50 range)
Pour Point: -11
VI: 122
Flash point: 440
AMSOIL Diesel / Marine 15W40 (overall ranking 750)(788 is the highest rating in the 15W40 category)
Pour Point: -47
VI: 164
Flash point: 453
What this data won't tell you, is what kind of wear capability the given oil has, and what kind of additives it has. Zinc is listed for some but not all the tested oils becuase some companies won't divulge this. In addition, the Amsoil Diesel / Marine has anti corrosion additives and additional suspension detergent capabilities which you can't really see on the can. The best way to evaluate is to look at the best data you can, and you won't find it on the can. You'll find it in test reports like the one on the link referenced above.
Regards,
Paul