i used
Amazon.com: Kat's 24100 100 Watt 4"x 5" Universal Hot Pad Heater: Automotive
i would agree with others, that larger heaters for a short duration would be more effective and efficient, but i've gone with a wattage that shouldnt come back to bite me if i forget having plugged it in w/o a timer in place. i followed the kats usage guide, which shows
Engine Oil Pan
2-5 quarts 100 WATTS
5-8 quarts 150 WATTS
8-20 quarts 250 WATTS
20-35 quarts 500 WATTS