I have a 2002 S430 - purchased about 3 months ago. The documentation and gas cap all say use ONLY premium but the dealer says regular is ok to use, any thoughts on this?

 

Sounds like the dealer is looking for extra business,no matter what it is I usually stick with what the guy who made it suggests not the salesman.

 

Your dealer is an ***. These engines are displaying use Premium for a reason.
91 octane is usually highly recommended; 89 octane MAY be used if you can't find premium and, IMO, 87 octane regular should never be used. The car will run but the performance and gas mileage will both suffer. IMO 93 octane is a waste of money but certainly OK to use.
Use premium 91. Your car will be happier.