What is your current oil pressure running, hot at idle and at cruising rpm?
That will have a large factor on viscosity of oil recommended.
The reason why the recommendation of Diesel oil, is for the engine additive protection. Namely ZDDP, which almost all modern oils have done away with. Even a lot of diesel oils curent formulation has diminished the percentage of ZDDP installed.
There are other oils out there, some fairly new additions, designed for classic and older cars that are coming equipped with additive packages.
I'm sure someone will chime in on Brad Penn oils, and Valvoline Racing. I myself use Yamaha motorcycle oil(Yamalube) as it still has the additive package in it.