Perhaps because the heavier relative viscosity of the oil when cold does more to reduce clearance than the thermal expansion of the piston? Though in theory a multi-grade oil shouldn't be much heavier cold, practice dictates that cold oil - even modern multi-grade oil - pours faster warm than cold, so perhaps the higher viscosity is helping minimize the slap when cold if enough oil is being flung up on the cylinder walls. BTW my NA 740 sounds as much like a diesel as my 780 turbo... I wonder which one will break first.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina