Matt has some great points to think about. Our engines are very dirty engines. Nothing like the modern ones now. And just need frequent oil changes as compared to newer engines. Some newer style engines use 0-20 oils. Syn's are great for these clean engines. With ours. 3,000 miles of use has pretty much ended the useful capacity of the oil filter. As at that point. Most oil is going to the oil filter bypass valve. Thus very little. If any oil is getting filtered. And the higher the miles on your Olds engine. The even shorter the life of your filter. You can reduce oil contamination some by installing EFI. The very best thing you can do to extend the life of our engines. Is simple drain the trash that's built up in our engines at 2500 to 3000 miles.
Mineral oils can take temps to around 310 degrees as I recall.
Syn's are higher. So what. Your engine is toast at 250 degrees. Syn's can be left in for more miles. But the filter should still be changed at the 2500, to 3000 range for our engines. I have a 96 Astro that's coming up on 300,000 miles with mineral oils used. And I never have had to add oil between changes. Would Syn's done better? All oils are to be change once a year. How many miles do you put on your GMC each year? Syn's for more modern engines. Have at it. In my GMC. I'll stay with the diesel blended 10W30 with 1800 PPM that I use. That's my 2 cents worth. You need to do what feals right for you. Bob Dunahugh 78 Royale