What kind of caulk to use around light fixture on masonite/composite/engineered wood siding?
The best thing to use is called "Butyl" caulk. It is mineral spirit based (meaning not water based) rubber like caulking so it will adhere to almost anything, it is flexible so it expands and contracts with temperature differences, and once dry, it can be painted. It can also be cleaned up with mineral spirits (paint thinner) if you make a mistake or get it on yourself. Butyl caulking however will last usually no more than about 10 years, so you must check it periodically and know when it is time to re-apply it.

Silicone based caulks will last longer than Butyl but cannot be painted and cannot be cleaned off of anything.