A few months ago I posted a question "will an oil leak that started
because I put synthetic oil in my motor stop if I go back to dino
The answers I got were not conclusive but some good thoughts from
different people were given, I thank them as always. Here is now my
answer to the question based on what my experiences have been. Leaks
my start if you use synthetic oil and will stop when you go back to
I have a 1981 300sd that is at 118K. It did not leak befor except for
an occasional small spot on the floor. I put 5W50 synthetic in to the
motor and a few days later I had spots of oil all over the floor. I
found the front and back motor seals leaking, the pan gasket oily
around the perimeter and dripping, one oil cooler connection with
small drip dangling driping always. Might have tightened it but did
not. The floor had to be cleaned every week.
3K later I did an oil change using Delvac 10W50 dino oil. The oil
leaks have stoped after a couple of weeks of driving. The small spot
on the floor that I used to see seems to still be there and it comes
from my oil cooler connection. Maybe I should tighten it now LOL.
My conclusion is that synthetic oil will cause leaks in some cars.
This my have to do with how the seals and gaskets were designed in
1981, or just the mileage. One speculation is that the seals of this
car have only seen dino oil. Dino oil swells some tings and synthetic
dose not swell them, so for 115K mile my seals have run swollen
against the crank and have worn to the proper friction to still seal
thins ok while using dino oil. When I used synthetic oil the seals
retracted and leaked. When I used dino oil the swelled back up. Same
principle applies to the old pan gasket. Note, I also put 5W50
synthetic into my 560sl at 96K and after 3K it has no leaks.