I've never worried about mixing oils. Petroleum, synthetic, and petroleum synthetic blend.
I'm loyal to either Spectro Syn-Sno, or Amsoil Interceptor.
But I've run Apv, Spectro Sno non-syn, Bombadier syn-petro blend, Klotz techniplate all without issue, all mixed in the same reservoir often at the same times.
I don't see much Klotz (clogs) in my future. Smells cool, inexpensive, synthetic, but doesn't make me feel as comfortable when I watch it pour into the reservoir at cold temperatures as the others I've listed.
Only thing in my experience about mixing oils that may be a negative, cleaning the power valves slightly more often may be necessary. Some of this could be that I have no problem pouring petroleum in my same reservoir as synthetic on pv engines.
During very cold temps, stick with one oil! Whatever it is you choose, stick with that for the cold snap.
Just my opinion... but...... 11k miles on my '89 Cougar 500, 7400 miles on my '94 Cougar 440, 9600 miles on '98 ZR 6, 9300 miles on '06 F6 that is still flawless, and currently 3800 on my '12 XF 8 and I have mixed oils in all of them without caring, except during very cold weather (below 5 deg F in my opinion)
Basically the best comment so far in this thread is the one above me, "The worst oil is no oil" if I remember it correctly.
Peace..... and oh yeah, Hammer Down brothahs !