I'm in AZ, where it has been known to drop below 80F in the winter once in a while. But I wouldn't even consider using anything thinner than a 15w50 or 20w50 in a motorcycle, especially an air cooled single or single crankpin v-twin. These engines put a heavy load on the con rod and crank bearings, and you need an oil with all the load bearing ability you can get. I have owned singles, parallel twins, v-twins, triples, inline and horizontally opposed fours, and a horizontally opposed six (Goldwing) I've never use anything but 20w50 in any of them, and never had an engine failure. Harley recommends only 20w50 in their engines.
I don't see any problem with any type of oil, conventional, semi synthetic, full synthetic, car, or motorcycle specific, as long as that last number is a 50. I'm using a very expensive full synthetic motorcycle specific oil in the RE, but I intend to follow the oil change intervals in the manual. I have a couple of other bikes I use cheap conventional 20w50 oil in, but change it every 1000 miles.
10w40 or 15w40 might work ok in a four or six cylinder engine, they do not have anywhere near the stress on the bearings as a single, especially a long stroke single like the RE.