When to replace motor?

if it aint broke, dont fix it. keep doing normal maintenance at regular intervals and regular oil samples and just stay on top of maintenance.

 

like country29 says, to which I add run synthetic oil ......

 

I ran a ford ranger 35 miles at 55mph with 0 oil pressure, never overheated, didn't lock up. Fixed the oil pump, and have ran the engine another 90k miles.
Mobile 1 synthetic.
Don't know if you could do the same thing with a truck engine, but I am sold on synthetic.

 

I've been told, switching to synithic oil with a lot of miles already on the engine will make it start leaking and burning oil.

 

I've been told the same but in the opposite. As in, if you use synthetic in a new engine it will develop leaks because the dyno oil is used to seal all holes and crevasses for the break in period. The synthetic is too thin and slippery for new engines, best used in older engines. Just what I've been told by diesel mechanics at a local shop when i switched to synthetic in my 7.3 f250.

Ethan

 

You are only spreading false rumors. Billy Bob's third cousin that lives over the holler doesn't know much.

Oil is not a sealant. The term dino oil is just dumb. Synthetic oil comes out of the ground just like conventional.