Ok, Doctor! I believe you have a chain saw, but I'm still not convinced about ATF in the Toro.
I also agree with your view on blending. I mixed a blend once of Castrol 10w-60 and GC 0w-30 - it gave me a 5w-40 kind of viscosity with a Group 4/5 composition, and the clincher was that both oils were approved by BMW for the engine. I ran it in my 2003 BMW M5 for about 4,000 miles. The UOA showed good wear results, but the threads on the lid of of the cartridge filter housing had a thin film of black goo on them. It encouraged me to try other xw-40 oils, but the mix was a one-shot wonder, not to be repeated.
It seems pretty clear that when it comes to oil, one plus one equals the total unknown.
Cheers
JJ