I went and looked at a single owner 2013 PIP last night, has 120k on the clock. Car overall looks quite nice, but when I pulled the dipstick there was hardly any oil and a bit of sediment at the bottom of the stick. I checked the service history via Carfax and it documentation shows a gap of over 50,000 miles with no oil changes. I took a look down into the engine via the filler cap and it looked quite clean. Not sure if I should punt on this, or move forward and religiously change the oil starting now.
And sediment or sludge is definitely a red flag. Parts of the engine may already have excessive wear from operating with clogged oil passages. One of the problems with continually topping off the oil level is that the crankcase oil keeps getting thicker as short-chain oil molecules are continually being lost and long-chain molecules are left behind to form sludge. With the drain plug out and the engine cool I would probe inside the oil pan for accumulated sludge. Any amount you find would pretty much be a deal-breaker, though you might go on to consider some oil flush products. Some of these work so well that new oil leaks appear where dirt had previously been making a seal. I have also seen sludge accumulate in a filler cap (not a Prius). You can imagine how bad everything looked when the valve cover was opened: everything was coated in thick, flaky black carbon. By that time anyone could notice that the engine did not sound right.
You will find that my 120k mile PiP has no record of oil changes for the past 60k miles. I stopped going to the dealer after 60k miles because they stopped sending me coupons. Sediment on a dipstick though... that's pretty bad when a shiny stick picks up stuff.
Same, I typically do all my own oil changes, so I was not too worried... but then I pulled the dipstick.
