valhalla360 wrote:
When you drain the oil, there is always a little left on the surfaces. With a diesel, it turns black due to soot. You just mixed the new oil with the old sooty oil. You just can't visually see that the soot is drastically thinned out.
As far as helping reliability, your average new engine will outlast the chassis just following the manufacturers maintenance plan, so there is nothing to gain by putting suspect used oil into the engine.
X2 to the soot issue with a diesel, same with my 2001 Cummins with 303K miles. Since I bought it seven years ago I have been doing 10K oil changes. Just needed to replace the tappet cover gasket at 300K and this what it looked like when I removed the cover!