I bought a new truck and it came with free oil changes. I knew I would never use their cheap drum oil in my vehicle I never took advantage of their "FREE" oil changes. I've bene using Amsoil in everything since the 70's and do swear my it. Anyway, one day I took the truck in for service and without asking me they changed MY OIL and put in their cheap whatever oil. I voiced my unhappiness but figures oh well I'll just run it till my next change what could it hurt. Boy was wrong. The truck ran like doggie doo doo. The engine was louder, ran rougher, I could feel more vibration in the steering wheel etc. I thought could it really be the oil? After 1,000 miles I dumped out the cheap drum oil and replaced it with what I致e been using and the truck instantly went back to normal, quiet and smooth with no vibration.
I noticed the same difference in my 60Kw generator when I changed it with five gallons of oil. My equipment runs in 120 degree temps all day long, last week it was 119 around here and my equipment runs cool in that enviroment.
Last story. In my Sprinter there are very few oild that are "approved" by Mercedes to use in their diesel engine. Mobil one is their designer oil of choice. I had the excact same situation as my truck, dealer changed it on my request as I was on the road and used mobil one. It ran lously compared to what I was used to using the Amsoil. Same thing, got home and dumped the Mobil one.
No I'm not an Amsoil dealer, just know what works for me with 50 year of changing oil in everyting from a water pump to large trucks.
Here's what I do when I take any vehicle in for service. I bring in my own filters and oil and have them change the fluids so I have it on record that they were the ONLY ones that touched my equipment. I've never been turned away. I do it with Ford, Mercedes, Kubota, Hino, GMC, Chrysler even quick lube places etc. They also change very little for the oil change when I bring in the fluids. The Ford dealer charged me $19.00 to change the fluids on my F450 diesel. They take their oil change "special" price of $39.99 take of their cost of the oil and filter and charge me the remainder in labor. For $19.00 and sitting in their lobby reading the paper and eating their cookies and coffee in a cool waiting room it's well worth it.