How do fan forced ovens work? How does 160* with a fan end up cooking food in the same time as 180* without a fan? I've never had a fan forced oven before so it is all a bit new and magical to me.
A fan forced oven works by forcefully circulating the hot air in the oven around the food, thus cooking it faster and harder - hence the need to drop the temperature (wind burn if you like lol)
A normal convection oven works on the basic premise that that hot and cold air are different 'pressures' as the two collide and mix, they swirl around the oven, thus cooking the food in a speedier fashion than, say, coals in the ground or a fire, because the air is coming from all directions, not just the direct heat on the bottom.

I have an oven thermometer, and when I have the oven on 160* fan forced, sometimes the temp is 160* or 150*...is it really a 20* difference, or should it only be a 10* difference?
Then the thermostat in your oven is not terribly good. You will find though that even in ff ovens, there are "hot spots" (when you cook a pizza there may be a side that always goes darker if you don't turn it, etc) so the temp will vary throughout the oven. This should be minimal compared to a regular oven though...

Are there fan forced ovens that allow for the fan forced/non fan forced conversion? Like, if I set my oven to 180* because that's what the recipe says, a fan forced oven will set the temp to 160* with the fan, even though the dial says 180*?
No - the temp you set is the temp you set.


HTH