I run Mobil 1 synthetic in my '95 Z28 6-speed now with 150k on the odo. Do oil changes every 6-7k and the old gal is still running great for her age (knock on wood).

 

The viscosity of the oil isn't going to have any other effect on a flat tappet cam than it will a roller. We use 5W-30 in our '66 Satellite with flat tappets. API classification will. And since most oils today are API SN (current) or SJ/SL/SM ('96-2011) and contain lower levels of ZDDP, that will have an effect. I believe they are roughly 800 ppm or less compared to the '60s 2000 ppm. That leaves two options: A) Buy racing oil that isn't API certified and has the higher level of ZDDP or B) Buy a ZDDP additive.

I spring for option B. You get the benefits of modern day oil with the protection the cam and lifters need.

 

Don't know what year or which engine the OP has, so the subject of ZDDP might be a moot point. But, I use one bottle of STP Oil Treatment in my flat tappet engines. According to STP, one bottle to 5 quarts oil gives an equal amount of ZDDP to what the older oils used to have.

 

I run Mobil 1 full synth in my 2011 camaro, and my wifes Dodge intrepid and our large Chevy Express 3500 family "Travel" vehicle. I am a firm believer. My dad owns a printing company with a fleet of vehicles, and he uses Mobil 1 on all of them, and some are over 500,000 miles now and still going strong. He also uses Marvel Mystery oil in all the transmissions. WIth that many vehicles and that many miles, he's doing something right and I am a believer.