Any marine (as in boating, not armed forces) store will carry this one. I'm on the East coast and I've seen it in 4 different chains for marine equipment stores. Seems that boaters have been using topsiders for years. It is about $39 which is about 3/4 of the cost of the dealer oil change. I changed my oil 11 times last year (28,000 miles with 230,300 miles overall).

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dtf
1994 E320 Wagon (Died @ 308,669 miles)
1995 E300 Diesel (228,000)
1999 E300 Turbodiesel ( died @ 255,000)
2006 Toyota Tundra SR5 AC 4X4 (115,000 miles) rusted frame - sold to chop shop
2011 Audi A4 Avant (165,000 miles) Seized engine - donated to Salvation Army
BMW 330 xi 6 speed manual (175,034 miles)
2014 E350 4Matic Wagon 128,000 miles
2018 Dodge Ram 21,000 miles