If it's the 3.4L timing belt engine, very easy on oil. Ime and imo, any 5w30 or 5w20 synthetic will work quite nicely. That includes but not limited to, ST Synthetic, QSUD, and Havoline Pro DS box to name a few.
10W30 Maxlife. Do not put a 5W nor a synthetic in the engine at 250K if it has not been run with it for some time. Run the Maxlife to 5K with a Mobil One M1-209 oversized filter. Leave the filter on for three oil changes. This combo has served me well over the course of several hundred thousand miles on the Toyota 3.4.
5W-30 or 10W-30 depending on the weather and a 3614/51348/90915-YZZB5 filter(an FL400s/3600/51516 will also fit space-dependent), a 5VZ-FE isn't going to complain.
FWIW, I ran a 1UZ-FE up until 297K with no issues on a non-HM 5W-30 and OEM filters. Running out of oil or not changing it enough is a bigger threat to a Toyota engine than oil and filter choice.
I'm running 10W-30 PP HM in mine with stock OEM filter. I flop between Pennzoil and Quaker State. Valve cover gasket was leaking a little so that's why I'm using the HM. Probably won't matter either way.
I've run 5W-30, 10W-30 and 10W-40.
Years ago I might have been afraid to change from conventional to synthetic oil on a car with a lot of miles. I don't feel that way anymore because most oils they call "synthetic" aren't really true "synthetic."
You said it doesnt leak oil..that doesnt mean it doesnt consume oil...
I would start out with Walmart or Costco or whatever 10w30 is the cheapest and monitor consumption before going with a higher cost oil.
Im sure you have looked it over nicely.. have you started it while cold does it smoke? Have you Pulled PCV while hot to see if it has blowby? Tailpipe soot?
Long test drive with multiple stops for CELs?
She's got a few miles on her!
I assume your location is correct, so this is a southern truck? If so, it should suit you well. 10w-30 should, as well, and at that mileage I'd just use conventional with a good filter. Plus, the A340E is a great transmission. Sounds like a solid truck, if the price was right!
Does buying a truck with 250k concern anyone but me?
Up here in MN? Absolutely. A clean, maintained, rust-free southern truck? No. A Toyota, that is. These things really only disappeared because they rusted off their frames in winter climates. Down south, no worries. As long as you keep up on the maintenance that thing will run for 500k, or more.