I had the same 350 engine with 460LE transmission in my 1996 GMC. Great motor.
It had 220k miles when I sold it, and was still running great. With that many miles, check to see also when the spark plugs were replaced.
My engine always seemed to like 5w30 or 10w30, in Mobil 1 High Mileage, Valvoline Max Life, and Castrol High Mileage. (in that order)
Also have your tranny looked at, if it is the 460LE. Mine started to act up with high miles & you may need to drain the tranny fluid & replace a screen. If it shifts erratically, you may need to also replace the Solenoid board inside the transmission...which rebuilt ones can run from about $80 bucks to $300.
GL