The recommended viscosity oil changes with ambient temperatures. If you don't know what ambient means go look it up in the dictionary.
As far as putting in heavier oil to mask problems, thats hack sh!t.
TSRM recommends 5w30 for temperatures of 50 degrees F to below -20 degrees F. Perfect for winter. I run 5w30 in the winter with no problems because my engine is not worn. If you have problems with running 5w30 in the winter, you better start saving for a rebuild.
TSRM recommends 10w30 for temperatures of 0 degrees F to above 100 degrees F. Perfect for summer.
TSRM recommends 10w40 for temperatures of 50 degrees F to above 100 degrees F. TSRM does not recommend running 10w40 in below 50 degree temperatures.
TSRM recommends 20w50 for temperatures of 80 degrees F to above 100 degrees F. TSRM does not recommend running 20w50 in below 80 degree temperatures.
I don't see any reason to run 20w50 in a MK3 supra unless the main bearings are excessively worn or the clearances are too big.
5w30 in the winter and 10w30 in the summer, period.
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