I had hellish trouble removing a base gasket that was baked on with 40 years of oil and heat. Modern paint strippers are bloody useless and I finally unearthed a tin of ancient Nitromors with methylene chloride in it. Even then, it took several soaks and scrapes but eventually it all came off.
Methylene chloride hasn't disappeared entirely, you can still buy it if you have trade need for it, but suppliers have tightened up considerably. After the new regs came in I could easily have got a gallon of it but I missed the boat on that by leaving it too late.