In your climate I don't think 23 degree minimum temp of 20W50 versus the expected 20 degree temperature is a big problem.
The problem with 20W50 is the fact that there is an inordinate volume of an additive called "Viscosity Index Improver" added to the oil to make a 30 weight spread from 20 to 50. This amount of VI additive is known for causing excessive "coking" or carbon buildup, particularly in smaller, hard working engines such as yours.
If you like Castrol products, and they are good products, you should consider using something with a little less spread. Even limiting the spread to 25, such as with 5W30 or 15W40 will make a dramatic difference in the amount of VI contained in the oil. To make that extra 5 weight spread takes much more VI additive, it is not a linear relationship.
Check out what is available, and also refer to the oil weight chart in your owners manual as recommended by previous respondents.
Best of luck,