Schaeffer, my primary brand for most of my needs, their 700 15w40 is a synthetic blend. A very good oil I use it in the warmer months, then go to their 9000 5w40 synthetic in the cold months. Using them in a 12.7L Detroit 60 factory reman with 606,000 miles on the reman. Uses 2 qt in 20,000 miles / 400 hrs. Never have to add oil between oil changes. I also use their 10w30 HDEO blend in a CUT and a zero turn mower and their 5w30 PCMO in a pickup and a gator.
It is true that both a 15w40 and a 5w40 will have essentially the same viscosity at operating temps. I operate my truck in the upper midwest all winter long and the 5w40 is a better choice for cold flow starts. If most of my operation was in the lower tier of the country, I would just use the 15w40 blend all the time.
Rotell, Delvac, Delo, all have 15w40 syn blends also, as they also have full syn 5w40's. Availability is different in different areas. I get a good bang for the buck using Schaeffer, but then I buy in bulk as opposed to the off the shelf. Pricing is competitive with anything on the shelf, and they give me free oil sample kits and analysis and deliver everything to me for free. No one else can match that deal.