I used my M18 transfer pump today to pump some 85W140 gear oil into an axle. I've pumped plenty of hydraulic fluid, diesel, engine oil, gear oil, etc with it over the years without too many issues. Today, for some reason, I had trouble with the output side being pretty turbulent and foamy.
My input hose size has always been 1/2". A year or so ago, I switched the output hose size from 3/8" to 1/2" and I'd like to think it helped, but today's work made it seem like it was a problem. I noticed that when I shut the pump off and let the oil in the output hose drain back towards the pump, it would let all the air gaps out and then typically work better once that side was "primed."
Today, I did not do something I typically do, which is set the oil out in the sun some hours before pumping it. I figured being late May in Arizona, I didn't really need to, but 85w140 is still pretty thick when not warmed up. This was perhaps part of my problem.
From a general fluid knowledge standpoint, should I have moved up to a 1/2" output hose or down to a 5/16" hose? Or is there something else going on here? The input side flow fills the hose and acts like you think you'd want it to, but the output gets foamed up and has air in it.