How many miles on this car? It would be very rare to have a leaking head gasket, or coolant leaking into the crankcase on the Series II S/C engine. The manifold is all aluminum, and the 03 used a much improved LIM gasket set. Has the engine been subjected to extreme overheating? If not, you can usually forget the head gasket as being a source of the leak. A good ole compression test & coolant system pressure test, would reveal any problems here.
If you are still concerned about coolant/water in the oil, you can use the simple; Water by Crackle Test. The result can be measured by placing a few drops of oil on a hot plate that is heated to 150° C. Positive water will bubble and crackle. This test will detect levels of coolant/water down to 0.1%. Its' only drawback is that any detected water may be from condensation as well as coolant contamination. The test is very subjective below 0.5%, however, this test is a good place to start. If positive results are found here, a more definitive test for identifying glycol is a chemical test. The test involves shaking the oil sample with a solution of chemicals and observing a color change. A positive reading in this test will be identified if there is in excess of 300 parts per million glycol present. If this test is reported as positive ethylene glycol, it should be taken seriously!
You should never use 87 octane in this engine! 91 is the lowest recommended octane and 89 should only be used in case of an emergency. I use 93 octane only! If you have been using a lower grade fuel than recommended, that could account for why; “sometimes when u gun the car the SES light will flash”!
In addition,
NEVER use “gear oil” in the S/C.
Use only GM S/C oil, part # 12345982 available at most GM dealers. It takes two 4 oz bottles for the M90 S/C @ less than 10 bucks ea..
Ingredients listed on the bottle:
Fatty acids, C5-10, esters with dipentaerythritol-phosphoric acid, (1,1-dimenthylethy)phenyl CAS-70983-72-1
diphenyl ester CAS-56803-37-3