I use MQL on my Bridgeport and out of respect for my lungs switched to cooking oil for a coolant /lubricant about a year ago. At first all seemed to go well but after a few months I became aware of a gummy deposit building up just about everywhere on the mill. Tried everything but could not shift it.
Google research pointed me towards Hexane as a solvent (its used industrially to refine veg oils) and although it makes short work of the raw oil the gum deposits are quite resistant to it.
A chance conversation with a Chemist gave the solution. He had a similar problem using veg oils as a lubricant in pill packaging machinery and spent some time researching it. The issue is nearly all veg oils oxidise to a gum at a relatively low temperature and the gum is so stable its difficult to dissolve.
The solution is to use a coconut oil, specifically fractionated coconut oil which has a lower viscosity and does not oxidise (turn into gum).
All clean and happy in M/C shop now and her indoors says I smell good enough to eat.
It seems the stuff is popular with the soap making brigade (as well as pill encapsulators) and I got a litre for 16 which will last me the best part of a year.
Cheers!