All 560SEL's have limited slip differentials - All 420SEL's do not.
I've had Michelin MXV4s, Dunlop D6A2's, Pirelli P4000's, all are ok in wet weather, I have only spun out while going up steep hills at low speeds or when really flooring it.
Don't forget, you're driving a 4000 lb. 208" rear wheel drive car - not a Honda Civic. It's not supposed to do that well in the rain.
2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino