Organic heat transfer fluids react (depending on their composition) to oxygen. That is an unfortunate fact.
For temperatures up to about 60 oC (140 oF), they oxidize slowly; however, above 140 oF they oxidize rapidly.
As a general rule, the rate of change doubles for each 10 oC rise above 60 oC, and the usefulness of the oil is substantially reduced.
Maybe you could look into non-organic synthetic fluids - but they are much more expensive.
Tim S.