Howard
of course one have to pay attention to every restriction. From an engineering stand point those resistances created by restrictions add up. So one restriction is not good , two are defenitely worse and so on.
The Setrab coolers have a 22mm inlet in which you screw in the adapter to AN 12
I modified this adapters before installation and opened them up to a max possible size of around 17mm, which matches the ID of an AN12 hose.
Same on the Oilfilter adapter, there are adapter available with AN 12 sized holes all the way.
Also on Fittings
There has been a comparisson between differend brand AN Fittings in the Hot Rod issue 09/12. They compared Sizes, Price and Quality ( no idea how they did this).
They did it on AN6 size. It was interesting to see that the ID of different brand AN 6 sized fittings differ from 0.220" ( redhorse, sniper) over 0.230" (Aeroquip) up to 0.265 ( earls, Fragola, summit)
TOM