Oil Sample low Viscosity

[Mango]

Hey All
My 89 burb 350 197898 miles oil samples coming back with lower and lower oil viscosity?
No coolant, fuel in oil sample?
New intake manifold Gaskets about 5600 miles ago
Burns oil only at start up valve seals
Could it be O2 sensor does seem to run rich $
How to check O2 sensor it is the original

Thanks

 

[Ricko1966]

Hey All
My 89 burb 350 197898 miles oil samples coming back with lower and lower oil viscosity?
No coolant, fuel in oil sample?
New intake manifold Gaskets about 5600 miles ago
Burns oil only at start up valve seals
Could it be O2 sensor does seem to run rich $
How to check O2 sensor it is the original

Thanks

I hate to just replace O2 sensors without testing them. But 35+ year O2 sensor,just get new one. While your old one is out put a volt meter on it heat it with a propane torch.watch your meter, voltage should be between .1-.9 volt and vary with heat. Another thing you could do is get the O2 hot,drive around a while. Disconnect the O2 sensor,put a meter on it,let the voltage get stable. Now create a vacuum leak,watch your meter. Spray carb cleaner in the intake,watch your meter. Let things stabilize. It should read .1 full lean .9 full rich .5 just right.

 

[fast 99]

200k is a lot of miles. Could just be leaking by the rings. Perform a leak down.