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Hi. 30 hg is milimeter of mercury, about 101 KPA. 26 hg is close to 88 KPA. That mean your vacuum pump is working. When vaporizing oil you want a very low pressure because it help to prevent needing too high temperature. You can't heat oil too much, it need to stay below the particular "smoke point" for the type of oil you have.
Oil is a complex compound. it is easier to know the evaporation by individual compound. Here is a list of various compound that may be present in it at 101 KPA of pressure:
You then have to find an evaporation table of temperature vs pressure for each separate compound in the oil.
Here is what that kind of curve look like for water:
If you want to know the temperature for "your oil" as a whole , you need to find the temperature of the hardest to evaporate compound. But at that point the operation would be pointless as a distilation process.
As for the elevation concern, this is only a matter if you are in an open environment to the air. If you are in a closed environment like in a vaccum, the absolute pressure given by the instruments will need to be used.