The clutch drum O-rings would require complete disassembly of the transmission to replace, so you'd do well to install an overhaul seal kit anyway. K2 applies in fourth gear, so if the O-ring in the clutch drum is hardened (probably is at this age) it likely will give you the symptom you describe with fourth gear.

What you want to do with the transmission depends on if you plan to keep the car for quite a while longer or if you're just looking to hang on to it until you can get another car in which case one of the "mechanic in a bottle" products MIGHT work for a bit.

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Erich Loepke
2010 Ford Focus
Currently Benz-less