As suppliments to central heating, oil filled radiator heaters are fine for safely heating small spaces, but before you buy any heater, you should first calculate the BTUs necessary to heat the GH you plan for, per cu. ft., insulating qualities, average temps and the temp you want to maintain inside.

ACF has a handy heating calculator on their website. Just enter the correct numbers per what you plan for and compare it to what any heater you are considering will produce. But don't leave the cost of operation out of it, because while electric heaters are cheaper to buy, depending on where you live and your rates, if you have access to nat. gas, it might prove a better choice in the long run.

http://www.littlegreenhouse.com/heat-calc.shtml

Those calculators on the little greenhouse website are really useful.
As far as electric heat is concerned, what are people's opinions of baseboard heaters? I was thinking about installing them along the 10' north wall of my planned greenhouse- the north wall will be conventionally framed and insulted (no glazing). I would just use these to heat the greenhouse above freezing during some parts of winter. Baseboard heaters are available for next to nothing at my local recycling center, though I realize the cost of the heater is a small portion of the heating cost!

I have a small 12" X 16" Hoop House I don't use It in the winter But start in early March for plants to set out in my garden zone 6 WV. I found that the oil filled heater you are asking about is not enough to heat my greenhouse. I use a kerosene heater at night and the oil filled in the day until the temps come up at night. This has worked for the last three years you just have to keep a close eye on your temps at night.

I use kerosene heaters. I'm in zone 7. On nights when the temps reach 25 degrees, I can stay near 50* with one 10,000 BTU heater. I leave for work at 6 a.m. On very cold mornings, I remove the tank and allow the heater to burn the kerosene remaining in the reservoir. This is called a dry burn and cleans your wick. I also have a fine brass brush with which I brush and clean the wick. It takes only 30 minutes for the wick to pick up kerosene and be ready to burn after replacing the kerosene tank. My greenhouse is 22 feet long and 9 feet wide. You will need a fan to distribute the heat. With two heaters at outside temp @ 19*, I can stay around 60-62*.

thought i would chime in here.. Last year I decided to use electric heat for the GH, rather than continuing to use kerosene. I purchased a Dayton (red) 240V 5600 Watt heater w/blower (check out ebay-cheaper). It works great. It has a built in thermostat so the heater and blow shut off when GH reaches a temperature and then turns on when drops below. I simply have a digital thermometer in my living room with the sensor going out to the greenhouse so i don't have to get up and keep checking. Even when the outside temps drop to near 15F, i can still keep the setting on low,and the temp in the greenhouse will still keep it in the low 40's. When it's windy i have to increase it more only because the air gets sucked out. I don't think i have ever turned the heat up more than halfway. It's a little more $$ on the electric bill not enough to go back to kerosene. Also I use an old metal electic fan to circulate the heat when it shuts off.
GOod luck and let us know what you do.