Freezer Fan Motor Oil Type

[Fridger]

I am restoring frequently freezer fans in refrigerators and using for some time used vacuum pump oil to lube it. I just placed the oiler in a freezer to see the viscosity at those temperatures and the whole column of oil got thicker. The tiny film of oil between the shaft and the bushing apparently has no effect to slow down the motor but I just want to be sure and not to give a chance to slow it down to a point it will heat-up the  winding more even still running and shorten the life of the motor or even burn out an electronic board if applicable. I do also put fan motors in freezer after refurbishing to test them and they spin fine once pulled out. I still would like to used the correct oil which will not drop its viscosity at such temperatures. I tried to google for oil for this application to N.A. Does anyone knows the correct type of oil they lube it by in factories when assembled? Thanks