Over the years, I've tried solvents, Simple Green, and Purple cleaner. Room temperature Easy Off oven cleaner beats all of them. Spray it on and you can almost immediately wipe off old dried oil with paper towels. If it's not coming off, leave it on 5 or 10 minutes and then wipe. If it still has a coat, you can spritz it with full strength or near full strength purple cleaner, but try with just the oven cleaner first. Like purple cleaner, if you leave it on too long it will soften the paint. Work on small areas of a square foot at a time. If it's more of a thick coat of gooey oil or grease, I often use a putty knife and scrape it thinner and then wipe with kerosene as my solvent. If the kero doesn't handle it easily, it's oven cleaner time.