My engine is a 4JJ1, in an Isuzu D-Max 4wd.
Factory oil is Besco Duramax 10w-30.
Other recommendations in the handbook are all 15w-40: Shell Rimula R3X, or R2 Extra or R2; Exxon Delvac MX; Caltex Delo 400 or Gold. And a few others listed.
I ran the factory 10w-30 oil for for about the first 2 years of ownership, and a DIY change of the oil every 10,000km.
Have had a Provent oil catch can fitted since day one, and would regularly drain approx 90mm of oil each change.
My local dealer stopped being an Isuzu franchise, and thus I lost access to the Besco oil.
So I changed to Delo 400, and have been using that since, so that's about another 2 years using Delo 400.
The Provent now captures approx 75mm of oil each time.
That's the only noticeable difference I have between using 10w-30 and 15w-40. The D-Max otherwise drives the same, the engine sounds the same and fuel usage is the same between the grades.