Own a chevy 92 (caprice). Keeps telling me to service the engine. Why?

My car keeps telling me to service the engine soon. Why is that? cuz I have changed every sort of oil (engine oil, brake oil, gear oil etc) and have even added the anti-rust water and the temperature of the engine seems to be okay yet it tells me to service it. Also, there was a leakage in the gasket and I got that fixed last night but the problem still remains - SERVICE ENGINE SOON. guys please help me out here I need a little advise. You guys think it could be the sensors that are playing with me? thanks.