It's helpful to remember that the POH minimum temperatures as posted in the C182T POH excerpt by Panzerbjorn are just that: MINIMUMs. Numerous other sources recommend pre-heating any time ambient temperatures fall below 32 dF, and some even suggest preheat is helpful for temperatures below 40 dF.
Regardless, preheating should be done carefully and with strict compliance with POH procedures. Most aircraft manufactures recommend against continuous preheating, though some companies that sell specific pre-heaters assert THEIR
products can be left on continuously. For example, the TANIS Aircraft Preheat System COLD WEATHER MODIFICATION OPERATING GUIDE, Document No: TPG0001, Rev. F at http://www.tanisaircraft.com/productattachments/index/download?id=32
recommends a minimum of six hours of preheating or continuous operation when ambient temperatures are below 32 dF.
In any case, single point pre-heaters (for example oil pan heating pads) shouldn't be left on continuously because severe internal corrosion in the engine can occur. Other pre-heaters have similar continuous recommendations. FWIW, I know of a very nice C185 that, because of severe pitting received a new engine after just one winter of continuous single point pre-heat.
Take a look at the Wing supplements to CAPR 66-1. Some of them speak to pre-heating.
Cold, think oil is just one of the issues with winter operation. Aluminum and steel expand and contract in response to temperature at very different rates. Serious cylinder scuff can occur following cold starts where engines are cold soaked. Low oil temperatures that congeal the engine's life blood greatly exacerbate the problem.
Preferred preheat is in a warm 40+ dF hangar. Warm air preheaters can also effectively bring an engine to temperature quickly (an hour or two depending). Big engines require more time to warm up than small ones, so don't assume 30 minutes of heat on a Cub will have the same beneficial effect on your O-520 Cessna powered ride.