Old gas makes some cars run like crap. I went abroad for a couple months a few years back and left my Honda in the garage for 60 days or so with a full tank of "fresh" fuel. Whatever the circumstances were, that car did very poorly on that tank of fuel. Hard to start, extremely rough running, and shockingly poor fuel economy. Fresh gas cleared it right up.

My SL sits a lot (also K-jet) and I found that adding Sta-Bil additive to the fuel really seems to help keep the gas from turning to green goo.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

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

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]