Yes it is normal for oil to build up in there. I run 40:1 through all my smokers without issues and it doesn't build up on the exhaust valve as bad. I clean my exhaust valves on my smokers twice a year. You can clean it while it's in there with some carb cleaner or remove the cylinder and all the parts to the exhaust valve in the cylinder and use some easyoff oven cleaner. The flapper parts of the valves will most likely have to be cleaned with a wire wheel as they are near the piston and the oil is burnt on rather than layed on from circulating in the cylinder.