How do I dab without wasting oil?

The perfect temperature dab that will maximize the amount of THC and flavor and not destroy your lungs is somewhere in the mid 500 degree range. If you don’t have a thermometer, the best way to gauge a good temp dab is to watch the dab melt into a puddle without much smoke until you put the carb cap on. The temperature dab will then most likely leave a little oil on the nail (which is appropriate). Don’t be so retarded that your nails turn bright red hot and sizzle to remove every last drop of dab. But you’ve burned off most of the THC and the nice smelling and tasting terpenes, so it’s almost as if the dab has lost its meaning.

Is it dangerous to use a silicone tub as a carb cap?

Silicone rubber comes in a tube. you mean carburetor and not starch or wheat? you mean cap as in cover or capacitor. I can think of a dozen ways to answer this question and half are not safe.