I have an 06 KR 150 with a 5.4 liter motor. Been missing, like it was only firing on 7 cylinders for a minute - changed motor oil - went to a synthetic, high mileage about 6 weeks ago.
Running really rough - changed the deep seated, 3 million $ spark plugs all the way around didn't correct anything - if anything its worse.
QUESTION: would changing to a synthetic oil cause this to happen? or worsen the firing problem?
QUESTION: If it isn't an oil problem where would I look next? fuel filter, fuel line - I'm at a loss.
check engine light blinking like a strobe light too.

 

Have you replaced the coil boots? Is your check engine light on or been flashing? I assume you'd mention if it were, but have to ask.

No, Synthetic oil will not cause a misfire or rough running.

 

It's been on but now flashing

 

What and where are the coil boots

 

The coil boots are in between the coil and the plug above the plug. You need a scan tool to retrieve the code that will or might tell what cylinder.

 

Certainly there must be a clue...

 

Thank you will check it out