Ummm, don't you work for/own this company?
I tried synthetic, ONCE, in my 108 4.5. The thing leaked like a sieve and I thought I had blown a seal somewhere. When I replaced it with regular 10W-40, the leak was gone and the engine ran just fine. Research suggested that the smaller synthetic oil molecules were able to get past worn (but not yet bad) seals. I have no idea if such was truly the case, as I have no oil analyzers or such.
Look at the evidence to date - your engine is 40-ish years old and has been running happily with older (40 year old) technology oil, and just-as-good 30 yr technology oil, 20 yr technology oil, etc etc. I have ALWAYS been suspicious of brand name supporters, mostly because I never gave a darn enough to really look at one recipe versus another, also because I never read Consumer Reports on the subject, and because I've bounced around from oil to oil (brand to brand) for years, in multiple Mercedes', a Fiat, a BMW, a Camaro, an MG and a bunch of other cars, and never with any issues.
If nothing else, follow the advice on 3000 miles per change, or less if it smells like gas.
Your engine isn't so special/unique or wonderful that it requires something as equally wonderful/unique ($$$$).