One of those questions that will go down a rabbit hole. Who can really know what is the "best" oil. I buy Costco 5w-40 and see no issues. Others may say straight 30 weight which I believe is what is in the Ford manual. Also somewhat depends on climate but not so much. Personally I doubt any good quality oil is likely to cause any issues. But for some this is a near religious subject so let the games begin!
Any oil is better than no oil. After that the temperature window affects the best blend. While it might not be the optimum blend it’s hard to be wrong with Delco 400 15-40.
The American Petroleum Institute (API) is the only national trade association that represents all aspects of America’s oil and natural gas industry. Our more than 600 corporate members, from the largest major oil company to the smallest of independents, come from all segments of the industry.
I just spoke to Brian at American Diesel a couple months ago because I had some differing documentation on oil capacity for my SP225 Lehmans and the dipsticks had been remarked several times. They still recommend Shell Rotella SAE 30. I find it at Walmart which is also the cheapest place I have found as well.
While I'm not going to recommend an oil weight or multi grade for your Lehman, straight 30 weight will be around for quite a while. I had a very popular ZF gear in my boat that required 30 weight oil. So I don't think 30 weight is going away any time soon.
Shell Rotella T SAE 30 was my oil of choice for the ZF gear.