Jay,
Fuel is frequently used to dilute oil prior to flight in cold climates.
There is actually a diluter system installed on some older Cessna's. It
will evaporate as the engine heats up.
Michelle

Jay Honeck wrote:

One of the best parts of owning an aviation theme hotel is getting to
meet folks who worked in the aerospace industry during the heyday of
American military aircraft development.

Today we met a fellow who worked for Pratt & Whitney on the SR-71
Blackbird's engines. He's here for a wedding, and is mad as hell at
his brother for reserving the "Blackbird Suite" ahead of him!

This morning he was talking about how difficult it was to get oil pumps
to work on those huge engines, given the incredible range of
temperatures and pressures they had to endure. In the end they had to
dilute the oil before every flight (Marvel Mystery Oil? :-) so that it
would flow at the (relatively) cold temperatures at start-up.

Later, in flight, the dilutents would boil off, leaving only "pure"
(and very viscous) oil. Thus, not only would they have to re-fuel the
Blackbird before each flight, but they would have to change the oil,
too.