Q: Gray oil under oil cap

There are quite a few labs (just do Internet search) that perform passenger car engine oil analyses and such a service is economical as it is done through the mail. Visual observation, in the circumstances you are describing, will not tell you much; instead, obtaining a lab report will tell you exactly what is in the oil. When I say, "your circumstances", you are reporting no change in coolant level; consequently, the most likely explanation for the gray color observed under the cap is condensation. If you desire a further, on site, inspection please feel free to request that using YourMechanic’s service request link. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.