Brett said:
The oil is caused by what is known as scavening affect plain and simple or should I say lack there of it when the headers are not sealing correctly.
Your problem is down to the exhaust gaskets you are using they are flat faced not round so are a lot harder to achieve an ideal seal esp on a Lav ...never could quite understand the sense in going down that route to be honest.
For me I use good old copper exhaust O rings which have a round profile which then crushes and does what its supposed to do. Nought any thing wrong with them plus the profile being round if the pipes arent dead square then they have some fudge.... If you get Honda 4 ones they are a bit thicker .
I have the same problem with my ?250 VW Golf!
No doubt the valve seals and guides are rubber ducked after 187,000 miles and some severe neglect (not by me I might add). If I go down a long hill and then open up at the bottom, there are whisps of blue smoke for a while.
I too favour the Copper rings gaskets.
I believe the seal is provided due to these seals crushing flat and thin, with the studs providing 'spring' to keep the pressure on when expanding and contracting.
As per Keith N's advice I smear Loctite Blue Gasket on the rings, to provide belt and braces sealing.
Even so there is a period when re-tightening is required, with the frequency stretching further out each time as the gasket flattens and studs 'load up'.
In general thin gaskets have a better track record of sealing than thick.
This is due to 'creep', which is the gradual compression of the gasket with heat cycles and thus relaxation on the studs/bolts. Having an Aluminium 'flange' doesn't help as they are weak as piss and bend with 'Gorilla Grip' tightening techniques.