I have 150w adhesive pad heaters on both my oil and trans pans. The max temp they can achieve is around 100 degrees on my 5 quarts of oil 4 cylinder motor. I don't think you will have to worry about overheating the oil unless you go a bit overboard on your heater. Mine are 150w because that is the size of pad that fits the flat part on the bottom of my pan. I'd think that if you just get an appropriate sized pad, then you will probably see a similar result as I have.
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