I have done some experimenting with different oils.
All cooking oils goes raunchy in 3 to six months
Walnut oil also go raunchy in 3 months.
The following oils develop bacteria that can cause food poisoning
coconut oil, peanut oil, walnut oil, sunflower seed oil and corn oil.
Mineral oil did not change and actually the wood had less bacteria on the boards after a few days with the mineral oil.
My niece used my test in her biology class in college last week and ran the cultures for me.