A rather notorious weak point in 996 Porsche cars is the rear main seal or RMS. A quick google search for Porsche RMS issues will return a ton of info on the problem. Bentley Publishers show you how to fix it however and if you have a 996 Carrera (Turbo, GT3, and GT2 996's with the GT1 engine block are not effected nearly as badly or often) the video is definitely worth watching and apply either as preventative maintenence or if you have increasingly noticed oil on the ground under your 996 Carrera.

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The actual process is not that difficult and you can do it yourself even with basic mechanical skill. All part numbers necessary and tools are listed in the video.