I have a 2004 Honda Odyessy with 125k miles.
Only “conventional” oil was used for the oil changes since vehicle was purchased new. Is it ok to switch to full synthetic oil (not a blend) without causing engine problems.
You can change back and forth at will. Just make sure the oil you use meets or exceeds the specifications for oil from the manufacturer (see owner’s manual)
Remember that switching to synthetic does not mean you can extend oil change intervals. I would guess on most cars it would be OK, but there are a number of factors involved in setting the oil change intervals, I suggest sticking with the OEM recommendations.
Assuming Honda doesn’t require synthetic for your van, is there a reason you want to begin using it? Is there something that OEM-spec’d dyno oil wasn’t sufficiently providing?
Other than doubling your oil change costs, what do you hope to accomplish??