motor oil will thicken due to temperature for example the oil weight rating 5w30. When cold the weight of the oil is a 5 weight and once warm is a 30 weight oil. 5w for when it cold so that the oil circulate easier on initial start up.
Well if its heated to a certain point, it will probably be thinner.
quote from wiki
"The Society of Automotive Engineers, usually abbreviated as SAE, has established a numerical code system for grading motor oils according to their kinematic viscosity. SAE viscosity gradings include the following: 0, 5, 10, 15, 20, 25, 30, 40, 50 or 60. Some of the numbers can be suffixed with the letter W, designating their "winter" or cold-start viscosity, at lower temperature.
Viscosity is graded by measuring the time it takes for a standard amount of oil to flow through a standard orifice, at standard temperature. The longer it takes, the higher the viscosity, and thus higher SAE code."