Hey guys... I found oil in coolant resevoir... don't know what it is, but it doesn't look like my head gasket blew. I dunno. The car still runs strong. It did overheat until i changed the thermostat. Now no overheating, but oil in resevoir. Do you guys know why? suggestions please... thank you..

do a compression test and a leakdown .... uasually oil in coolant beans a blown head gasket but it could be something else .... check the oil filler cap and dipstick to see if theree is a puss like substance (thicker then the oil in yout car) its uasually tan .... itf there is a lot of that then its probably your gasket