[Antennas] What to put in a dummy load

[Jim Isbell]

I have a 200W dummy load that apparently once held oil. It is now empty. 
At least that assumption is made on the basis that the bottom of the 
container has a bellows loaded bottom to allow for expansion.   There is 
no place to fill or drain it but I can cut a hole and add a filler cap 
real easy.  Once open I can check for leaks and repair them.  OK, now 
what is the best thing to put in it...or does it really matter that 
much.....Corn Oil is cheap and readily available.  Is there any real 
advantage to mineral oil?  Or is there any real advantage to some exotic 
concoction that I never heard of.  What about a fan and lots of air, and 
no oil at all.?