What oil are you running in your gas tractors

I have ran Mobil Delvac 15w40 in everything for years, except the gas Ford trucks I have. They get 5w20. I have put the Delvac in newly rebuilt engines and some that were worn completely out. Gas or diesel. I have had no problems at all. As said already, I think any oil would have to be a lot better at lubricating than what they had when these old tractors are new.
I even run the 15w40 in the transfer pump engines, welders, generators and everything. It keeps from someone grabbing the wrong oil when something needs some.

Well, we only have 2 gas burning trucks. Most are diesels. I drive one and my son drives the other one on the farm. The 04 F350 with a V10 has 245,000 miles, and it still won't use any oil between changes. He drives it on the farm and won't let me get rid of it. I drive the 12 model F350 about half the time. It's got 138,000 on it now, and it never uses any either. So it's not as bad as having to add some to something and getting it mixed up if someone else drives one of them.

Zinc additives for lubing solid lifter cams is much more of an issue with high reving engines with stiff valve springs to allow the high reving.  Not so much on engines with valve springs designed for 2000 rpm operation.

However, if you want zinc, the 5w-40 Mobil 1 diesel oil has 1100 or 1200 ppm zinc.

For the fleet at work from little trucks to bucket trucks to backhoes, we use Castrol 15-40. At home in the Tractors and equipment I use 10w40 Cam II and in JoAnna's CTS Caddy I use 5-30 Cam II Synthetic. In my Astro Van I've always used 5w-30 as required since it was new. That is also Cam II or equiv.