I don't know what kind of a vehicle you drive but you say you want to go away from the manufacturer's required 5W30/10W30 oil so I would assume you are driving a fairly new vehicle. In the USA most gasoline engine cars, SUVs, vans and light trucks have required 5W30, 10W30, or even 5W20 oil for a long time now. So I don't know why anybody would even want to put 10W40 oil into a modern car or truck. Oftentimes in owner's manual's it is stated NOT to use 10W40 oil.
In the case of my own car which is a 2003 Saturn Ion in my owner's manual it is stated that 5W30 is preferred, 0W30 can be used in extreme cold, and 10W30 can be used (warm temperature only) if 5W30 cannot be found. The first Saturn car I even owned straight 30 weight oil was allowed in warm weather but now straight weights are not allowed. In the case of most gasoline engine Ford and Honda cars and light trucks 5W20 oil is either requirred or preferred. I know a woman who bought a Honda Civic and in her manual it stated that 5W20 was the preferred viscosity but 5W30 could be used if 5W20 was not available.
It is best (especially if a vehicle is under warranty) to use what the manufacturer requires. If I owned a newer Ford pickup truck I might be willing to use 5W30 in the summer (if it was allowed in the owner's manual) but I would use 5W20 in colder weather. I sure as heck would not stick 10W40 into it.
It fact, I don't see a lot of 10W40 oil on the shelves. It is mostly 5W20, 5W30, and 10W30, with some diesel engine oils that might be 15W40 or 5W40 and some oils for European type cars.
If 10W30 is allowed in the summer for your vehicle why not just use 10W30? And then you could use 5W30 in the wintertime.
