As for the filters, I doubt you can get them at Pep Boys, or Auto Zone etc. Just go to the top of the screen, click on Fastlane and look your car up. You will find a Hengst filter for $6.39. This filter will come with a new seal for the drain plug.
Now as for the drain plug size, I believe it's 19mm I don't remember. I do know it's huge compared to the 13mm drainplugs on my diesels.
Now as for the whole synthetic vs. dino. oil goes. I don't think that anyone has documentation of how much longer an engine will last using syn. vs. conventional. Just change the oil religously.
Now, since my 911 is solely dependant on oil (ok, air too) to keep it cool and lubricated, I think what Porsche has to say in the owner's manual speaks volumes about oil types and weights. The owner's manual for my Porsche does not state a different oil change interval for synthetic, vs. dino oil. It also says at the top of the oil chart page that the recommended weights are based on temps, regardless of the make up of the oil. I think that if synthetic oil made that much of a difference, one it would of been factory fill at the time. The factory fill was Shell 15W-40 BTW. Two, there would be 2 seperate charts and change intervals listed for the 2 types of oil.
Simply put, syn. cost at least twice as much as convential oil. Now, will your engine last at least twice as long? There are people who put 350,000 miles on a MB V-8 using reg. oil. So, would using syn. make that go at least 700,000 miles, I don't think so.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member