Time, temperature and oxygen promote hydroperoxide - thin films left on press/die are more prone to oxidation than bulk, oxidation rate doubles every 10C
Just as a side note. I worked with a stamping machine in a automotive plant having trouble with sludge in summer time. Turns out stamping oil - mineral oil based - had hydroperoxide building in summer. Addition of 100 ppm BHT did not help, determined original mineral oil had hydroperoxides so addition of BHT was after the fact. Although BHT stabilizes oil against oxidation, it produces hydroperoxide as by-product of its reaction with oxygen or peroxy radicals. What level of hydroperoxides are you measuring? Is there any antioxidants in your oil? Also did work with peanut oils in making potato chips - Vitamin E in oil in chip was neccessary to keep chips fresh during storage. Again during summer - Vitamin E (tocopherols) were depleting during shipment from South and hydroperoxides were starting to build greatly shortening shelf-life. Potato goes in 30% water chip comes out 30% oil - reason they never have to change oil.