There is nothing special that needs to done to start using synthetic oil. I think its more important to pick an oil with a good additive package and
Heavy Duty Engine Oils are recommended for flat tappet valve trains by the
Corvair Oil Guide. See
Engine Wear. An excellent synthetic HDEO is
Rotella T6 5W-30 and I like semi-synthetic
Petro-Canada DURON SHP 10W-30.
Nondetergent oils were used when Stovebolts were new so any modern multigrade oil would be vastly better. ZDDP is the antiwear wear additive in engine oil and its actually the phosphorus that provides the antiwear function. OEM-style valve trains don't need much more than 800 ppm of phosphorus and amounts over 1400 ppm are not beneficial. See
Engine Oil Myths - GM TechLink.
If your engine uses much more than 1 quart between oil changes, I would be more inclined to stick with a lower cost conventional HDEO.
See
YouTube: Is Synthetic Motor Oil Good for Everyday Cars?If your fuel economy suddenly dropped, the cause should be reasonably easy to determine. It's not clear whether you already have an HEI ignition system or are thinking about upgrading. HEI is an excellent ignition system and is very reliable, especially if the ignition module is kept cool.
