a question for sl6 engine sages- i have owned a number of sl6 powered cars over the last 40 years and have noticed common characteristics among them mostly beneficial and a few negative. the positives mostly have to do with the unusual durability and longevity of the engine. some fellow owners and i were discussing the fact that none of us had a sl6 that ever leaked oil from the rear main bearing seal. this represents about 25 sl6s from 1960 to the present, most being very high mileage. we also had ford chevy and amc 6s and most ofthem developed an oil leak at this point by 100k miles. none of the sl6s did. we were at a loss to explain why the sl6 avoids this problem and what the different design may be. years ago the sl6 club news did an article analyzing from an engineering standpoint why the sl6 was more durable than other 6s, but the rear main bearing oil seal was not reviewed . does anyone have an opinion and reason for its durability or is our collective experience only a quirk and not representative. thanks bob f

The rear main seal on my D-150 starting leaking at about 120,000 miles. It has bled oil and stained my driveway ever since then. The one on my Duster also leaks oil, but not as bad as the truck. After a long drive the tailgate of my truck will be covered with oil splatters.

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2

Yeah, I've got a 1963 170 that leaks oil from the rear seal as well. It's going to be fixed sometime in the near future though.

When compared to other engines, the SL6 rear main seal is large.
Fact is that it is the same seal that Mopar uses in the 440 and the 426 Hemi engine.
When set-up correctly, they last a long time unless the engine is run dry of oil or grossly overheated.
DD

Would synthetic oil or oil treatments cause an original seal to deteriorate?