I know many are against it, particularly since it gives you cancer in California but Xylol (xylene) works great on adhesives. We have used it for 20+ years. Just have to be aware of a couple of things. It is fine on modern automotive urethane paints but do NOT ever use it on the thin polyester paints on trailers, RV's, etc. Also generally fine on fiberglass gelcoat. Avoid getting it on plastics, especially lenses for any sort of lights. We soak a rag with it, then keep the adhesive we with it for a minute and scrape it off with a plastic blade. If even in doubt about the paint on a surface, always test a tiny spot first to make sure but we have used it on well over a thousand vehicles with modern paint jobs including industrial single stage paint like on city buses without any problems.
I've also used it on bare aluminum but if the aluminum is coated, just test in a small inconspicuous spot just to be sure it will not affect the coating. You can get it at most hardware stores, paint stores, Lowes, etc by the gallon usually for under $20. Also makes an excellent degreaser too. It does have a strong odor and solvent resistant gloves is recommended as it will really dry you hands out and is probably not good for your skin.
On the topic of removing reflectives.... you get what you pay for. The cheaper the reflective, the more easily it bruises on install, harder and more aggressive on install, tears easier, and much harder to remove. We print a lot of reflective since we do so many police, ambulance, and fire service vehicles. For any of the larger agencies that turn vehicles over pretty quick, we almost exclusively use 3M IJ680CR as it almost always removes as clan and easily as a high end wrap vinyl. Yes it costs more up front but when you are also removing 20-50 police cars a year, the savings on the back end more than makes up for it. It took me a couple of years to finally convince some of the larger police departments of this as they remove the graphics themselves on retired vehicles. Once they started seeing it take them 30 minutes to strip a patrol car versus 2 hours (and still leaving all the adhesive and often also scratching the paint jobs all up) and seeing higher used resale prices on the cars since the paint jobs were much better they now require 3M IJ680CR for all of the city fleet vehicles. I just wish it was not so expensive. On newer clients it takes quite a bit of effort to convince them why spending so much more than the cheap options competitors are pushing is worth it.