Vanguard Twin 16-HP

My lawn tractor is powered by a B&S Vanguard 303447-1147 that is about 15 years old with about 700 hours on the engine. It has performed flawlessly mowing about 2 hours per week – until this spring. Early this spring I serviced the unit and changed to the recommended synthetic oil for the first time. As I began using the tractor, I noticed that it was throwing a LOT of oil. I discovered that the dipstick tube seal-ring had deteriorated – replaced it and the o-rings on the dipstick cap and valve cover oil fill cap.

However, the engine continues to blow some oil through the breather assembly as confirmed by the oil collecting in the air-cleaner base, but in much smaller amount (about a pint every two hours of running). I've replaced the breather assembly (PCV). The leak-down test is about 8% and seems to be the same for both cylinders. I've checked the valve seals, and the left side cylinder has a seal on the exhaust and intake (I don't know why). The right side has the valve seal only on the exhaust valve (as it should).

I continue to believe the engine is still sucking air via a crankcase leak. I'm aware that any small air leak can cause this, but am unable to locate the source of the problem. Any ideas?

2 things.
"1" why did you switch to synthetic oil? That engine was never designed with the idea of using THAT oil. Sorry but the whole synthetic oil debate bothers me. In my shop, everyone seems to think that switching over will solve every problem and cure cancer and make them rich. All by switching oils. Sorry. I feel better now. I'm trusting that you did your homework on the matter.
"2" seems/sounds to me like a headgaset problem. The gaskets will leak between the cylinder and pushrod hole. This may not show up on your leakdown test, but that is better that a compression test. Try your leakdown again with the engine hot, valve covers removed and pushrods off the rockers. This will allow you to test with the pistons at bdc, which should also tell you something about your cylinder walls.
I think this may actually be 3 things.
Hope this helps.
Tim

By a name brand 10W40 oil and try it again.(I kinda like Shell Rotella)

Thanks for the two replies.

Tim, I did immediately change back to SAE 30W at the time I replaced the dipstick seal (I initially blamed the problem on the Synthetic oil). Not to argue the point, but the Briggs owner"s manual for this engine actually "strongly recommends the use of synthetic oil", but I"m not a fan. I"m to the point of just pulling the heads and visually inspecting the head gaskets as I have wasted a lot of time trying to diagnose this problem to no avail. The thing doesn"t start pushing oil until she"s warmed up.

Re-did the leak-down test and the right cylinder is the culprit. Tore it down and removed right piston. Compression rings still within spec but the oil rings are shot. Got the nasty surprise that the pricey ($33) rings are no longer available, and you have to replace with newer, pricier ($90) piston set. Maybe a short-block is in order...