I remember a time when the Harley dealer told me a similar story about using synthetics. "Use only Harley oil" they said. Then it was "synthetics are too slippery and will cause problems with the roller bearings" they said.
Then a few years later, Harley started promoting their own synthetic oil and started using this from day 1 in their new machines. Interesting.
I use synthetic oil in everything we own: 2 cars, motorcycle, lawn mower, generator, chainsaw, string trimmer thing - and when it's time for my 50 hour service, it will also go into my 2310.
I'm NOT an Amsoil dealer (and in fact use Mobil 1 more often than Amsoil, just because it's easier to find in rural Maine), but I have heard stories about Amsoil going to bat for consumers when someone threatens to not honor a warranty because of their product. I'm also not an attorney and can't cite case law - but statements like this are all over the Internet in just about every user group.
But it's my understanding that a company cannot force you to use their brand of lubricant, or spark plug, or whatever. I think if there is a problem, you need to document what you used and when, but beyond that the company would bear the burden of determining that an inferior lubricant caused the problem - and I'm sure the Amsoil gang would be very interested if a manufacturer made such a claim about their lubricants.
But the bottom line is that it's your machine. Use what you want. It's well documented that one of the hardest times on an engine is when it is first started and the oil is in the pan. Makes sense to me that if the oil can flow at a lower temperature, it will get into the engine quicker.
YMMV, but I'll be ordering some Amsoil as soon as I can find a dealer - or I'll set myself up as a "preferred customer" and order it directly.
Jay
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2005 Saab 9-5 Arc (0W-40 Mobil 1)
2003 Toyota Camry (0W-30 Mobil 1)
2003 Harley FLTHCI (Amsoil in engine, primary, and transmission)
1998 Generac generator (0W-30 Mobil 1)
198? Jonsered chain saw (Amsoil 100:1 oil)
2008 Massey Ferguson GC2310 (< 20 hours so far

)
'nuf said....