High mileage oils usually just have extra additives to help old seals, slight anti-wear changes and things of that nature.
No matter how beat the horse is, gotta accept that oil discussion never ends. What I’ve gathered - T6 is a good oil in general, and I’ve always felt like part of the reason it’s chosen is simply because it’s one of the widely available 5W40 options… good cold starts, but a slightly heavier weight at temp for certain vehicles that might like it. These engines, like many “older“ 4x4s, are often called tractor motors for a reason. They are relatively simple and are not built with the high tolerances of today, where you really should stick to a 30, or even 20 weight in newer vehicles.
It’s often mentioned that Toyota recommends 5W30, but that’s just the EPA recommendation in the US under the idea of fuel economy/etc. Toyota’s global spec for these engines is anywhere from 5W30 to 20W50, depending on climate. I don’t know that 5W40 was an option back then, so in my mind, it sort of bridges a gap that might be a sweet spot for many users. Again, good cold starts/colder climate, while retaining a more robust 40 weight at temp. There’s also 0w40 these days.
As mentioned above, “diesel” oils have additional additives to help deal with the dirtier nature of diesel engines, but many are approved for gas engines, and it’s not going to hurt anything. Some feel it might clean a dirty engine too much at once and cause issues, but I don’t know that I’ve actually heard of that happening? If the engine is that dirty, there are probably bigger things to worry about. There’s also the whole topic of ZDDP/antiwear additives and many oil analyses coming back showing 40 weights to protect a bit better against wear in these engines.
And the idea of age > wear > slightly lower oil pressure > slightly thicker oil. I have been using Mobil 5w40 “turbodiesel” partly for that reason.
You can really use whatever you feel happy with, just make sure it makes sense for your climate/use, and perhaps most importantly that you HAVE oil in there