Transformer Oil Level in Degree Centrigrade

Yes. Oil expands as it gets warm, and contracts when it gets cool. 35°C is used as the standard temperature for volume measurements.

The standard temperature for gasoline volumetric measure is 60°F, if I remember correctly.

The indication is not for level at that ambient temperature, but rather the normal level when the OIL is at that temperature. A power transformer in operation produces a significant amount of heat, such that the oil temperature will normally be well above ambient.

To add to this, you will need to do a survey on your transformer to find the exact time of day to when it is at its highest temp rating. A heat sensor with a digital input to a computer would gather your information for you with physically having to point a pyrometer at it. Then once you know when the average time is, you can physically go and check your level of cooling oil, then you will have a handle on when it is necessary to file the unit and keep it at its optimum performance.

Unless the unit is very small (<2500 kVA) an external heat sensor on the casing would not help. Casing temperature will vary with many environmental factors (sun/cloud, humidity and wind speed, to name but a few). Those factors are insignificant compared to the heat generated internally by the transformer losses, which are mainly dependent upon transformer load. You may get more reliable data measuring temperature of the radiator cooling fins, but that's still not as good as checking the temperature of the top oil.

Loading is often consistent from day to day, but usually not over longer time spans. For industrial facilities, the load profile may shift on holidays & weekends, and also if a major load is out of service for any reason. Residential and commercial load profiles tend to change with the time of year due to weather (heating vs. cooling) and lighting (shorter/longer daylight cycles).