You will have a bit of old oil left in the system if you just drain it and change the oil filter. My guess it is a 5qrt system so 10% would acount for 1/2 qrt. May not seem like a lot but your crankshaft, valvtrain and pan can hold residual oil that may account for your 10%, also sludge and other deposits too. Now the way you actually change oil, at a dealership or higher end service stations (lol not jiffy lube) they use a cycling system to send in fresh oil through the car and pump out the old. Once the color changes or my guess the computer on the device says the oil is completely clean then all the residue is gone and everything is good. This is wonderful since it removes sludge and all sorts of gunk out of the engine. You may never get 100% clean oil on the engine but honestly if you like doing it then by all means please do; you are one of few americans who take the time to do so and its something to be proud of.
The Factory Man
May, 10, 2008 AT 11:49 AM
**The Oil Life Monitor DOES NOT sample the oil.** It calculates the oil life based on a very complex computer algorythym and it monitors miles driven, temperatures, drive cycles and several other sources of input. I am not sure why your system only shows 90% when you reset it, mine showa 100% for a very short time on my 2005 Silverado.