Oil + additive viscosity

Yep the restrictions are big problems IMHO. A 5/16" (8 mm) opening has only 40% of the flow area of a 1/2" opening, and around 50% of the flow area of a 7/16" opening. And it is probably more restrictive than that, once turbulence effects are accounted for.

Also, the brass adapters in and out of the filter mount can be restrictive; I've drilled those out to 1/2" before. I can't see if the filter mount's hole are a bit small or not, but they may be too.

Regardless of all else, IMHO you need to get that 8mm restriction out of there. I am not sure how that particular remote mount will work without it however. Is this remote mount made for this engine size, or a smaller engine?

Do the filters you use have a check valve built-in? Ours do (just a rubber flapper). So perhaps no check valve is needed in the mount here if you can get that filter type. I'm a fan of remote mounts but never have put one so high (I guess your steering box is in the way of a lower mount), and am very careful about avoiding any restrictions.

Oh yeah, our /6 standpipes don't have check valves in them.