On the 20-50 Castrol.  Yep polymers do change with heat.  The base oil is around 7 grade,

with cold polymers will change to 20, when they get hot around 50.   I managed to overheat my 12A rotary that I did get over 280F (pegged gauge) oil temps.  Motor did "get me home" but was trashed from excessive heat. When the oil was drained, it came out like water!  Next morning the engine wouldn't hold water either (aluminum went beyond critical temps... ). So hopefully someone can check with a petroleum engine on the polymers and critical temps (my data point is over 20 years ago, not sure if still valid with today's oils)

 

Other 'seemed like a good idea at the time' was to use Castrol R - hemped based oil that worked great in our two stoke motorcycles. Three Mazda's in the rally circuit, three blown engines.  Live and learn.

 

I'm on digest mode, so appreciate everyone trimming their replies. 

 

Tom