Many would agree that Pontiacs engines can have rear main seals that leak. 4 spd cars can have rear main seals that are a bit more prone to leakage than automatic cars. There are varying degrees of leaky rear main seals. The experts say that synthetic oil can cause some seals to leak more that may have sealed with conventional oil.
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I have a rebuilt 400 with about 1000 miles on the engine. Engine is in 78 TA 4spd. The engine was assembled with .002-.003 clearances on the main and rod bearings. Initially, I ran 10w-30 conventional oil. I was noticing that the rear of the engine (by the rear main seal) was leaking a very slight amount. I would get the occasional few drops of oil on the ground after parking overnight. I could see there was a bit of oil in the 4spd bellhousing. I could see that the back of the block by the rear main seal was a bit damp and oil stained. I am pretty sure the oil pan gasket / oil pan seal is good. So, I suspected my rear main seal was leaking a slight amount.The 400 has a BOP rear main seal and I followed the instructions. I changed over to 10w-40 conventional oil (from 10w-30)and this seemed to improve the leakage from the rear main seal/rear of block. I was getting less oil dripage and the back of the block was not as damp.
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I really like all the benefits of synthetic oil, but I am not sure about how it would work in a Pontiac rear main seal that is a bit leaky.
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Questions
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1. What are people's experience with synthetic oil and rear main seals in Pontiac engines?
2. Would it be recommended to use synthetic oil in a Pontiac engine ,with a 4spd, that has rear main seal that may have a very slight (ie. occasional drop or few drops of oil from rear main seal if parked over night)?
3. I see different viscosities used for synthetic oil (looks like most run 10w-30 to 15w-40). What do experts recommend for synthetic oil viscosity in normal (ie. engine assembled with .002-.003 main/rod clearances)Pontiac engine?
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