Unless you put together your own engine, own dial bore gauges and know every clearance in your engine, don't just guess and put any old oil in. Listen to the builder.
That being said not all 10-30 20-50 5-40have the same pour values either. 4 different 20-50 oils by different manufacturers can have a different pour value, which will alsoeffect how it gets thru the motor cold.
Do what works and has been working for ya. my .02
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