Synthetic experience
I have run the same brand of synthetic in most of my machinery for the last 14 years. My cars (5) have 1 year oil change intervals (with normal filter change intervals); motorcycles, outboards, and even my air compressor get the appropriate oils and lubes. Additionally, I provide motor and transmission oil for a friend who races a Street Stock dirt track car (season champion). Last year he ran 16 races on the same oil - unheard of in a situation where oil is normally changed every race or two. The engine builder could not believe the condition of the bearings (used them this year too) and other wearing parts when the engine was torn down. Diesel-specific oils of the same brand frequently go over a hundred thousand miles between changes, using special "by-pass" filters that take out extremely small contamination particles. Yes, it costs a lot more per quart, because it is "built" to a performance standard, not down to a competitive price ($1.50 - $2.00). You can spend from $3.50 to $7.50 a quart for a synthetic. If it is my engine, it gets the very best (and most expensive.) Yes, it will find weak spots in gaskets - it penetrates better than petroleum oils. If you keep it clean, you can run it almost indefinitely, because the molecules don't degrade. The best benefit it gives, in my opinion, is not the fuel savings nor the extended change intervals, but the added life of the engine.