transmision oil

Assuming it's an automatic. Not sure what you meant by "0m cool", but, if you check it before it's warmed up, you will be showing a little low on fluid. As the fluid warms up, it expands, and once fully warmed up, will show accurately. You CAN check and add fluid when it's cold, but DON'T overfill it. Once you have gotten used to how much difference there is between where it reads when "full hot", check it when it's cold. Now you know where to fill when cold. Not sure if this answered your question, but if not, let me know and I'll try to nail down a more appropriate answer.