Update lang mga sirs, I changed my rad cap to a federal mogul brand with a 0.9 psi rating, but still nagbabawas ng tubig sa reserve tank.

I just noticed the other day that the water inside my radiator has some slight rainbowish color, it looks more like a rainbow swirl to me. Could this be a sign of a busted cylindrer head gasket? Should I replace it ASAP?

I'm not overheating naman. Sometimes my temperature goes up above half but (a bit higher than the midway point but not up to 3/4) goes back to half after a few seconds.

I popped up the hood a day after I noticed the rainbow swirls and took the radiator cap off, I used my flashlight to take a closer look inside my radiator. The presence of the rainbow swirls was not present but I saw some tiny specks. It doesn't looks like rust to me. I also checked the water inside the reserve tank but there are no rainbow swirls in there.

How can I know if this is really oil in my radiator? I need you help badly before I make some drastic moves. Thanks!
1