Your owner's manuals show the recommended viscosities for the temperature range you expect to see prior to the next oil change.
My '88 owner's manual shows 20W-50 as suitable for all temperatures from 32F to the highest expected, which should be suitable year round in Saratoga. Interestingly 10W-30 is only listed as suitable for temperatures from -4F to 50 degrees.
My manual shows 10W-40 as suitable from -4 to the highest expected, but GM, in particular, does not like 10W-40s because they have the highest concentration of viscosity index improvers, whose oxidation products they claim increases combustion chamber deposits.
These recommendations are specific to mineral based oils. Synthetic oils have a broader VI without the addition of VI improvers or with very little VI improver.