My Mercury decals were starting to chip & peel off so I decided to put new ones on. Removal was easy but what is the best thing to use to remove the old adhesive?

3M makes an adhesive remover that works on just about anything. I've also seen a couple of things in Home Depot paint department that claims to do that also.

heat gun and gooff

Goo Gone is the thing to use! Apply, wait 5-10 minutes, take a putty knife and apply 2 layers of paper towel across the edge, then bump along across the adhesive. It will roll up on the paper towel. Still takes some time and elbow grease but not as hard as I thought.

Use a hair dryer to soften the adhesive. It won't get hot enough to blister anything, but it will do the job. Once it gets hot enough to blister, there's no going back.

Ditto Hughes Marine.

acrysol can get it at any paint and body store.

Any auto body shop will have a rubber wheel that will remove the decal and not harm the paint.

Wd-40