I generally switched to synthetic oil in my cars (5W-30) and my diesel truck (5W-40). My farm equipment, I still use 15W-40 conventional diesel engine oil. I would have to say the reason I have not switched the farm equipment is because I do not operate that equipment in the extreme cold weather in the winter. The tractors and the combine typically are run, beginning in March on into November. I do not feel I have much to gain in that equipment, since most of the time, these engines are started up and run for a lengthy duration during their operation. I do like the improved "pumpability" of the synthetic engine oil in cold conditions, but again, it is pretty rare the tractors and the combine are started and operated at temperatures below even 30 degrees F.
With that, I do like the improved starting of our automobiles and my diesel truck during the winter months. I do believe the engines turn over and start better with synthetic versus conventional oil in the crankcase. I bought a 4 year old car back in 2007. I traded that car for another car in late 2015. That was the first car I started using synthetic oil in, and all but one car has been switched over to synthetic engine oil. I never had to replaced the battery in that first car, and I suspect that may be due to the easier starting of the engine. As I recall, the battery in that car still was the original battery. It is looking like I am needing to change the batteries in my diesel truck as they are now 10 years old, and likewise in another car I had purchased, which is now 12 years old.
I also believe the synthetic oil is a bit more "forgiving" in running over on an oil change. My wife and daughter do not watch the oil change tags closely enough, so their cars are likely to be run past my typical service interval of 5000 miles, which tends to follow more along the line of severed duty. Having change engine oil in our cars and my truck, I appreciate the thinner viscosity of synthetic when I have changed oil in very cold conditions, by seeing how it pours out of the jug.
Do I think synthetic oil is a better choice for our vehicles? Yes I do. Do I think the life of an engine has been significantly increased through the use of synthetic oil? I cannot say for sure. I have one car approaching 300,000 miles on the odometer, and had another we got rid of when it had 312,000 miles on the odometer. Both have had conventional oil in them since we originally purchased them used. Neither really consumed oil to any extent. The one we have now has always leaked a bit, but it has been less than a quart between changes. I haven't had another car we kept quite that long, and used synthetic oil in the engine. I haven't driven any of our vehicles in "hard driving" conditions or pulled a trailer to any great distance to really put an engine oil to the test either.
I switched to synthetic and I am satisfied with the results in our vehicles. Perhaps some day I might in my farm equipment, but not at this time.