What do you check before deciding to buy a used truck?

russianrullete.... all diesel engines are made to run with a diesel grade mineral based oil. If it was run on synthetic and switched back to mineral it would be fine. Someone has fed you a line of bull. Most of your better synthetics are fully compatible with mineral so if you are low, you can grab a gallon of the first oil you can get and get the levels proper. That is more dangerous to the engine than mixing grades of oil. There used to be a few brands that were not compatible but I dont think they are available anymore....

 

I was always told you couldn't go synthetic then back to a non synthetic too. But then they came out with the blends, so would think they kind of negated their marketing approach when they did that?

 

Again that was the old logic before the oils were made compatible. It was like you were not supposed to mix a heavy detergent based oil with a non, almost instant sludge machine.... that is in the past now. Almost all oils, including our fuel, ULSD, are a mix of mineral and synthetic. Its how they can control viscocity breakdown to give longer intervals between changes and in fuel, how they lower emissions.

 

although mixing them does no real harm to the critical parts they are supposed to protect, dont' they turn milky when you do mix em'?

 

When I traded my 98 KW to a Pete dealer the dealer dynoed my truck before he would tell me how much he would give me for it. That should tell you something. You would have to pay for a dyno but it is money well spent. Just think of the repair bills if you wind up buying someone elses trouble.
I would also have the fluids tested. Bring a creeper with you when looking at trucks and get under it and have a good look at the drive line too.