Originally Posted by Guitarman

I must step in to say that cured urethane enamel has no "oils" whatsoever, so none can be depleted. This is just one myth that has been around forever, and you can't really blame anyone for saying it because not everyone understands the chemical nature of car paint as it currently exists.

The solvents that exist in the makeup of the paint off-gas for a while when curing but when fully cured, urethane enamel is a solid film that does not blush or emit anything. It is in essence, a form of PLASTIC.

That said, anything highly alkanline or acidic can over time, dull the finish, it won't happen immediately though.

The only immediate negative of using dishwashing liquid is that it strips wax and most sealants (to varying degrees) so you're kind of shooting yourself in the foot by using it regularly, unless you wax/seal afterward.

Hope this helps.