I use the in-pan oil heater from Stef's on a big block Chevy wind machine to pre-heat oil before use in cold conditions. They also sell heater pads, and I have found these do not transfer the heat as well. But they work well under batterys. We use a lot of power to turn over engine with 12 blade 6 foot diameter propellers and cold batterys won't cut it.
To look at the Stef's heater go to
www.Stefs.com/33.htm
Thanks,
Randy