I’ve read many old threads regarding Rotella. Isn’t it the T4 15w-40 for gas engines, or T4 15w-40 for heavy duty diesels that folks are using?

The 15W-40 for diesel engines. It is supposed to have more ZDDP than 'gas' engine motor oil. If you have excessive blow-by and still have a cat underneath, it may cause issues due to the additional detergents in the oil not playing nice.

I don't have any blow by, but do have a cat

No motor oil contains any ZDDP, anymore. Diesel oil contains more zinc but ZDDP has been discontinued by all oil manufacturers. ZDDP is also harmful to cats, for the same reasons it was beneficial to the engine internals. Just use any quality brand of oil, 10w30 or 10w40. You'll be fine.

This is incorrect. When oil manufactures refer to "zinc" they are actually referring to ZDDP, which is zinc and phosphorous. Several oils on the market still have ZDDP. Valvoline VR1 oil is one example. That's what I use in my engine.

Matt

With the exception of racing oils, what I stated is correct. VR1 is a racing oil.

Racing oil is still motor oil, is it not? Many of the so called "racing oils" are street legal for normal everyday use in older flat tappet engines. Amsoil, Mobil 1, etc...are other examples.

Matt

Being in Phoenix and never dealing with cold weather, should I aim for a 10w-40 or 20w-50, rather than something lighter?

Shell produces T6 Multi-Vehicle for fleet owners that have both diesel and gasoline engines.
Per Shell:
"In order for a diesel engine oil to meet API SN, Shell ROTELLA® T6 Multi-Vehicle 5W-30 is blended with lower levels of phosphorous so it doesn’t damage catalyst systems in gasoline engines. Even still, Shell ROTELLA T6 Multi-Vehicle 5W-30 Full Synthetic has demonstrated excellent wear protection over millions of miles of testing in heavy duty diesel engines."
With no catalytic converter, however, it is OK to use the straight T6.

I also use the Valvoline VR1 motor oil in the 304 in my '79 CJ5. I use 10W-30 and change it twice a year.

I don't put many miles on my 5 - Maybe 2 to 3 thousand a year.

I had used vr1 10w-40 until they discontinued it, now I use Brad Penn Penngrade 1. Claims to have high levels of ZDDP, but it's pricey.

I've been using Schaefer 20W50 which is recommended for flat tappet engines. Was recommended by the engine rebuilder too.

You are correct, however it is a specialty oil. While it is currently, widely available, it isn't necessarily available, everywhere.

You are correct. It is, however a specialty oil. While currently widely available, it isn't necessarily available, everywhere.

I ran regular Valvoline 10w30 for three years, in Phoenix and Lake Havasu with no ill effects.

So we agree, many oils still have ZDDP, unlike what you said in your first post. As for availability, that's what Amazon is for. Free shipping, and it usually arrives at your doorstep 2 days after ordering.

Matt