I know lower weights (5W or even 0W) are good for cold weather climates because they are thinner. Heavier weights are thicker and better when pulling heavy loads. as far as synthetic oils or brands of oil. I have no preference; however some mechanics, I know use synthetic, more expensive but better in the long term for the life of your engine.
You can use any kind of 10w-40 oil for a nighthawk
Baby oil
Most use Synthetic Oil.
Rocket oil
They use the oil God gave them in their oil glands.
synthetic oil
What are you substituting the oil with? What kind of recipe? You couldn't use butter for the oil in salad dressing. Sometimes it does matter what kind of shortening you use.
fully synthetic oil
Olive oil.
engine oil
fully synthetic oil
mineral oil