I'm with you. Oil in at the top. The load is on the bottom, the cams push the tappets up. So the bushing needs to be plain and smooth on the bottom to support the load.
If you have a twin, MAKE SURE you get oil into the injector pump support casting on the side opposite from the governor end. This area is lubbed by throw off from the cam bushing. That is not enough to lube the injector pump cam and roller tappet. Several posters have reported worn out fuel injector tappets / cams in this area.
I ran an oil line to this area from the oil pump: I tapped a small oil line off the top of the oil pump, and also drilled a hole in the injector pump support casting, aimed to the cam / tappet. I tapped this 1/8" pipe. I modified the 1/8" pipe end of the hose fitting by soldering in a short piece of steel brake line. When I threaded this fitting into the support casting, I bent the stub of steel brake line to aim the oil to the contact point of the fuel pump cam and tappet. I put a Frantz toilet paper filter in the oil line for good measure.
Scott E