WD is not a lubricant. Don't care what it claims on the can, it is NOT a lubricant.

You can try disassembling the old door check and greasing it inside with silicone grease (silicone grease repels water, will not turn to "glue", and does not attract dirt). If that doesn't sound like something you want to mess with, replace it before it seizes. There is a very strong spring with a couple of balls that drop into detents. It relies on a grease film to slide. Creaking comes from a lack of lubricant and an ingress of debris and rust.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)