STP contains two significant ingredients:
1) a viscosity index improver, keeps the oil thicker at high temperatures

2) zinc dithiophosphate, a great oil anti-oxidant and a wonderful wear and friction reducer for iron and steel part (think piston rings on cylinder walls, and gears driving camshafts and injection pumps).

The first one is bad for diesels, because it tends to make varnish on hot parts, like the underside of pistons, top piston rings in diesels, exhaust valve stems, etc.

The second one is good for diesels, makes the oil last longer and cuts down wear of many internal engine parts.

Red STP has less of the viscosity index improver and more of the zinc dithiophosphate, compared to blue-bottle STP.

Dr. Lee