Read a lot about oils for SL, now I'm more confused.

Dealer say 5W30
Some people say 10W40
Some say 10W50
Some say 15W50.

PLEASE HELP

THANK YOU

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1984 928S
1991 BMW 850 6 speed
1991 BMW 850 Automatic
1986 Mercedes 560 SEC
1988 BMW E30 Convertible manual