oil and smoke coming from exhaust pipe

Hi all, I have a 83 kz1100 and just just recently started having oil and smoke spitting from the exaust pipe after only riding for a few minutes. I am really hoping its not the piston rings so if anyone has any ideas that would be much appriciated! I suspect it may be the valve seals but I'm not sure as there was quite a bit of smoke coming from the exaust and the back of the muffler and inside of the pipe is all oily. Just did an oil change and replaced it with 10w40 blend motorcycle oil. Number 4 spark plug is oily and wet so oil is getting into the cyllinder obviously. 

It does sound bore related, stem seals would normally cause smoke on overrun etc. Do you have access to a compression and or cylinder leakage tester

If it has been standing for a while it could be gummed up oil scraper rings.
If it's been rebuilt it could be over lapping oil expander ring.
Other option is valve stem oil seal or head gasket leaking into the bore from the oil gallery. 
As its actually enough to get into the exhaust and not burn off I would guess oil scrapper/expander rings issue. 

I don’t have a leak down tester and it was running great just yesterday did a 40mile trip so I’m hoping I didn’t overstress it. I did a compression test before I bought it and it was good but I will do one again just in case. The guy I bought it from said he did a leak down test and it passed but then again he could just be saying that. 

Darn I was hoping it wasn’t one of the rings but it sounds like it might be. I have been riding regularly so it hasn’t been sitting a while which makes it all the more frustrating 

If it's been ok and regularly ridden then the likely suspect is a stem seal which is the easiest to diagnose and oil in the exhaust would indicate an exhaust valve seal as oil from a leaking inlet generally get burnt first.
Let the bike sit overnight and pull the exhaust headers.
If the exhaust stem seal is leaking it will be obvious as the valve head and stem will be wet and you will see where oil is getting past the guide.

 

I took zed1015's suggestion and took off the headers this afternoon. the header for #4 was very oily and had lots of carbon deposits when compared to the others however it did not seem wet as I would have expected it to after sitting for nearly 24hrs. I included a photo as well

Another odd aspect is it only seems to start smoking and spitting oil after riding for 5 or 10 minutes.

After doing that I ran a dry compression test and all the cylinders were within about 20% of each other although the highest I got was 100psi which is slightly concerning but I won't worry about that for now. I don't have access to compressed air for a leak-down test but I feel as that may be the next option to troubleshoot.

Last quick update, I ran it with the filler cap off and there is a LOT of blowby, nearly no suction into the filler cap

wrote:

Last quick update, I ran it with the filler cap off and there is a LOT of blowby, nearly no suction into the filler cap

Before you get too deeply involved I would suggest checking the crankcase breather is clear, a blocked breather (sludge, Carbon etc.) would cause it to build crankcase pressure like this. Also check your air cleaner as crankcase pressure will push oil into the air box through the breather

Thanks to everyone for all the great suggestions! I will check the crankcase breather as Wookie58 suggested and have arranged with a buddy to use his supplies to do a leak-down test as well. as someone pretty new to motorcycles I really appreciate all the help!