A straight with 500,000 miles is okay if it's a class 8.
Most class 8s will go 750,000- 1,000,000 miles w/o major issues. I sold my last class 8 with about 950,000 miles on it. If I was with a different carrier I wouldn't of even considered selling it.
I easily may have gotten another 3-400.000 miles w/o major expense.
For those skeptics: N-14 ,10 spd manual.
Pretty bullet-proof.