This is the bike I have now, bought it in 2016 for 6k
Do you see the large section cut away from the top of the
oil tank that makes room for the mono shock? If you add too much
oil it seals off the two sides.
This bike was built in 2007, the builder just used it as a show bike and never rode it because of the way the
oil tank was made. There was only one vent on the top left side of the tank. If you added more then a quart and a half of oil it would seal off the right side of the tank and cause pressure to build up causing the cap to blow off and oil to spew out.
The fix was to remove the oil tank and have a second vent added to the left side and I also replaced the filler bung and cap with one that screws on.. (My buddy did the work, I have no talent ) then of course powder coat the tank...
I hope I explained it well enough. I don't have much talent at writting either.