I run an electrical contracting business and I service a lot of air-cooled 4-stroke engines as well as water-cooled and diesel engines too.
About 5 yrs ago I did a ton of research on different oils, mainly based on the zinc issue with flat tappet and older engines as I service and maintain both.
As far as the synthetic's vs regular, my only experience with true full synthetics is, I really never seen much good or bad in them. Personally, the full synthetics are expensive and if your properly maintaining your engine......I don't see the big advantage.

My only recommendation on non-synthetic or blends is to stay away from any 10W-40 oils, there is too much of a spread for the polymers to make it a good durable multi-grade oil.

My only recommendation is to stick with the manufacturers recommendations for weight and viscosity, along with operating location temperatures be it synthetic or not.

In my research, I found basically only two regular oil manufacturers with the highest zinc content:
Shell Rotella 15W-40 is one, but it's a specifically rated oil for mainly diesel engines and depending on your operating location temperature and being a 15W oil it may not work well for you.
The only other oil manufacturer who uses a high content of zinc across the board in there oils is Kendall GT-1 and it's a synthetic blend not a full synthetic and I have found over the last 5yrs to be a really durable oil, with minimum break down. I've also been using there diesel rated 15W-40 Super D-XA oil with excellent results.
I can't say much on oils like Brad Penn or Joe Gibbs, etc.
Only full synthetic I've ever tried was Amsoil and didn't see much advantage to using there product for the added cost, etc.
There is one other full synthetic 5W-30 oil that is an excellent product that I use, I have no idea who makes it or what there spec's are, as it's a highly guarded secret, but Briggs & Stratton's full synthetic works great.Even being a Briggs dealer, they won't tell us anything other than it's a 5W-30 and full synthetic, oh and it's green in color. Maybe someday I'll figure out who makes it.

Mark D.

1968 W200 Sweptline
1969 D100 Utiline
1971 D100 SE Sweptline
1993 D250 Club Cab Sweptline
1986 W250 Cab & Chassis
1978 D300 Sweptline

E. Brookfield, Massachusetts