You can use any oil that you would normally use for cooking but they tend to go rancid over time.

Plain mineral oil is commonly used. You can buy it in the drug store (it's unscented baby oil).

If you're looking for more of a film finish, there's Behlen's salad bowl finish.

Pure Tung oil (not the blends available at the hardware store).

If i remember correctly only mineral oil is FDA approved.

Are you looking for a counter top? If there is any sharp objects that would come in contact with the surface. I. E. a knife and cutting board i would dare use a film finish. Just my 2 cents.

Pure tung oil, walnut oil, mineral oil, in my order of preference.

OTOH, the chance that you would eat enough of any finish from having finished a countertop with it to cause a problem is about zero. As Paracelus tells us, the dose makes the poison.