Found this.
YR keeps saying he's old, not sure he's this old though! :D
Does a good job of laying it all out and why the timing does actually matter. Something interesting is that super wide mains clearances help get flow to the rods, but with a high revving race motor there's reasons to not run super large clearances too. This is where grooves come in.
Seems to me the big issue is that trying to feed oil "down" into the crank is that the oil transfered has to "last" until the journal passes TDC on two cylinders before getting replenished and there's not a lot of oil supply in the passage during that time, and even less when the transfer window shrinks.
By feeding "up" the oil can be arriving when its needed and so its less likely to be depleted by the time the greatest need arrives.
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