I continuously circulate the wort in my mash tun during the mash, even tho I use an electric-heated HLT/HERMS system, still like to ensure consistent mash temps throughout the grain bed. The new march pumps from MoreBeer rock. Buddy had the older style, and it is amazing how much better the new ones prime.
No problems with oxygenation issues. I simply have a dip tube passing thru a hole in the false bottom of my MLT, as is typical.
This connects to "Pump#2 IN". Pump#2 OUT connects to ball valve at top of MLT. Inside the MLT this is a simple hose laying atop the grain bed, similar to how the guys at theelectricbrewery.com suggest.
No issues with channeling, or anything else, and efficiency is between 80-85%.