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I'm not sure the relevance of discussing M2 N55 LL-01 UOA and B58 usage, the N55 is supported, B58 hasn't been since I believe 2020, the old versions of ISTA-D show it generically being supported, but the updated BMW info only shows up to 1 litre of top up with LL-01 or SN Xw40 oils, not a full fill.
Perhaps I wasn't being pedantic enough when I mentioned "weight", too colloquial vs being accurate, viscocity is what I had previously referred to. Higher viscosity oils at a given temperature will flow less volume for a given pressure and with the same heat transfer coefficient will spend longer transfering heat due to the lower fluid volume and will hence increase in temperature more than the higher volume flow from the less viscous oil, besides any fairly minimal shearing effect on heating.
In most engines you are hitting the oil pressure limiter well before the red line, hence lower flow with a more viscous oil in that scenario.
In the B58 with a 0w20 oil I have no reason to doubt the suitability (assuming of course a stock engine, anything else is outside of the engineering test envelope), it definitely is not suitable for the N55 (M2 or otherwise), as it was never engineered for that oil. The HTHS has to be matched to the engineering of the engine which will depend on bearing area, clearance and oil passage sizing, in itself it has no absolute meaning, otherwise we would all be running 10w60, as an Xw40 HTHS looks inadequate in comparison.
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