I read a bunch of the old debate threads and I agree its not needed when doing an oil change. Not even looking to start that debate! I generally run too far between oil changes and sure enough when cold weather hits I have a problem with the HPOP reservoir draining, will run about 10 seconds and I have to refill the reservoir to get it to restart. Decided I'm going to switch to a full synthetic since I suck at regular maintenance.
Going from either Delo or Rotella (can't remember which one I used), should I drain the HPOP and heads when switching to the synthetic? Not sure if its bad to mix the two, even if its only a couple quarts vs 15.
I would fix the issue of loosing the oil out of the HPOP. As for changing the oil in the reservoir, that's really up to you. Since you're switching to synthetic, I believe you'll want to change it pretty early for the first change. So, I would still change the oil in the HPOP reservoir, but I don't think it is necessary other than the thought that you got more of the old oil out.
To get all the old oil out, you need to pull the motor and take it all apart. There is probably a couple more quarts hanging around it there! LOL Seriously, it says right on the bottle it OK to mix with dino.
You said now that its cold your HPOP drains down.....Well when oil gets cold it gets thicker and doesn't drain very good so.....Synthetic will not cure your problem, just make the drain back issue worse.
Have you checked your oil level lately? if these things are low on oil and its real cold they will suck the pan dry before the cold, thick oil drains back to the pan. I've done it many many times here in the frozen north.....It was -25º when I was heading home last nite. mine died twice while warming up at work.