Temp differences

Good morning fellow hoarders, can anyone answer why would my temp controller coming from the OWB HX would be so different from the temp gauge on the oil boiler? There right next to each other. Usually it's a 20 or more degree diff

 

Does the water from the wood boiler circulate through the oil boiler?

 

I'm going to say you are circulating hot water from the woodboiler through the oil boiler! The oil boiler likely weighs around 300 to 400 lbs so the temperature drop is trying to keep that 400lbs at 170 degrees.
Fifty years ago, with our first wood boiler, it was plumbed in series with the oil boiler. Doing a little experimenting I found that just to keep the oil boiler at 180 degrees for twenty four hours would take 350,000 BTUs a day to keep it at temperature.
It is a little bit of work and design but it is more efficent to plumb the boilers in parallel.

 

Or keep it in series but have isolation valves so you can bypass the oil burner.