Depends....does your air cooled have a catalytic converter? Is your available street gas ethanol blend or are you even running E85 for some mods you did?
Or are you driving a pure factory pre cat car?
Best to follow the API Service Categories for your specific vehicle and use.....one size does not fit all.
API-SJ may be perfect with its high HTHS. But maybe you need high Zinc......

DRIVEN GP-1 pros and cons
 

Those are great points that get lost in the “exhaustive prolific oil thread” ether.

3.2 very warm climate, Steve Wong chipped, 91 octane, catless, no track just street. I think this is a very common iteration.

By all accounts, VR-1 is an excellent oil formulation, and it has the huge advantage of being available at most FLAPS for about $10 a quart.

Re synthetic versus conventional oils: The petroleum oils of today are a much better blend of base stocks than 20 years ago. Do full synthetic oils last longer? Yeah, but how often do you change your oil? Remember that with 12 quarts of oil, a 15,000 mile oil change is equivalent to a 7500 mile oil change in a big V8 engine that holds half as much.

Several years back I decided to just do oil sample analysis and change oil on condition, rather than by arbitrary time or mileage. So far, I've found that in several of my applications, the oil is still in good condition well beyond the recommended change intervals, so I'm also extending my change intervals. So far, so good.

The advantages of synthetic versus conventional is not mainly the increased change interval. Although that is an economic benefit. The main advantages are wear protection and the ability to survive better and function appropriately at higher temperatures.

Don't forget that half the reason to change your oil is to remove contaminants that build up from the combustion process, which arguably build up faster in older engines like 911s. Synthetic oil can't improve that. Worth considering before you leave dirty oil in your engine longer.

Scott, yes, synth oils can tolerate higher temperatures without deteriorating better than petroleum base stock oils, and that's an advantage in racing or other high-temp conditions. Like running the AC in Arizona! Here's another little known benefit of synth oils--they have about 10% higher specific heat than petroleum oils. That is, they are better at absorbing and carrying heat away from the internals, which is another argument in favor of synth oils in air cooled engines. And that's one of the reasons I use them.

stownsen914: If you do oil analysis, the lab will report the TBN remaining in the oil. This is a good indicator of how contaminated your oil is. Then change on condition. If you are someone who changes their oil every 5000 or 7500 miles, then you can do fine with petroleum oil. An oil change of Driven is about $200, and a lab test is about $30. I'll do the lab tests several times before changing the oil. Besides, I want to know what's going on inside my engine, and oil analysis provides a great tool.

If I had an air-cooled 911 street car, I would be running Mobil 1 15W-50 in it. I would never run a conventional oil with a full synthetic oil like Mobil 1 15W-50 available at less than $6 a quart...

Does synthetic reduce the volume and harmful distribution of byproducts compared to dino, assuming both are combusted in equal amounts ? Perhaps, if that is the case, the syn can improve on that front hence extending the oil change interval. However, I believe neither can improve on the non-combustion contamination process from a poorly tuned, driven and/or poor fueling system.