i use one of these and got some info for you,
these things thermometer are located directly on them and they create a sort of heat bubble around itself and your thermostat can claim 75f on it but really itll be only like 66f in the rest of the room, they also drop humidity alot so try to get an idea what your humidity is during the off time.
i recommend to buy a thermometer from home depot and wire it up (BE SAFE AND DO SOME RESEARCH FIRST) and then have it near the plants canopy, these run around 50$ and are not technically necassary but a little fan blowing heater with a thermostat built in would work better. also if you got a small fan that is unused you can keep it on low and have it blow above the oil heater about a few inches and itll given better heating coverage.
if your oil filled heater lacks a thermostat and is only a low-med-high setting heater than id recommend getting a thermostat for sure, also what size is your grow area? you do not want that heater next to the plants as itll roast there leaves, a small fan blowing heating is what i recommend.
mylar also will not melt as long as its not within about 12inches.