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20-50 oil is pretty thick when cold, and the pressure will go above what the oil pressure bypass can handle, and leaks from seals can and will happen.
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Hi Glen, where did you get this info from? The oil pressure relief valve will handle the common oil pressures even at winter temperatures below 0 °C without problems. Many 40 years old engines without a rebuild do often leak, thats normal. And often at parts/areas of the engine where the maximum pressure wont even be present like oil return tubes bermuda triangle etc.
The only thing where a 20w50 isn't the best choice is at cold start at very low envioronment temperatures like in NE winters. But once the engine is cold running enough pressure is given so even oils with higher viscosities will get to the parts where they're needed. And with a cold engine a careful engine warmup with not exceeding 3.000 RpM i.E. is only needed not because of the oil and its viscosity but cause of the chracteristics of the thermal expansions of the specific engine parts like P&C, Heads, Valves etc.