This is a common problem with the Honda Odyssey van and it along with the solution are well-documented in the VCM Mega thread posted in the OdyClub forum at https://www.odyclub.com/threads/the-vcm-mega-thread.367894/.

These vans will turn off some of the cylinders at high speed in order to improve fuel economy. This feature is known as Variable Cylinder Management (VCM). However, there is a design flaw in the piston rings which allows oil to bypass them and enter the cylinders. This leads to fouling of the spark plugs which in turn leads to misfires in the affected cylinders. The dealership will want to replace the spark plugs and install new high tension piston rings to fix this charging thousands of dollars for this repair. However, it can be fixed cheaply and easily by replacing the spark plugs and installing a device (VCM muzzler) which disables VCM.

Ironically, after I installed the VCM muzzler and replaced all my spark plugs with NGK Ruthenium HX spark plugs (purchased from Rock Auto), my MPG actually improved which I attribute to the spark plugs which are supposed to be more efficient. On the old spark plugs with VCM enabled we were getting 21.3 mpg over ~8000 miles and with VCM disabled on the new spark plugs we're now getting 23.1 mpg also measured over ~8000 miles.