Like you, I'm no oil expert. I got my feet wet having to learn about oils suitable for turbocharged engines.
At that time (2003) A3, B3 was the specced oil for the Saabs in the family and Volvo turbos too if I'm not mistaken.
I believe the "A" had to do with the oil's ability to withstand heat and the "B" and the lesser known "C" had to do with dirt suspension and additive longevity.
I didn't memorize this stuff nor did I have to as proper oils were available.
Much like using the French made, 3 prong Volvo plugs in my non-turbos. They made it easy. Just buy the right thing.
That being said, don't hold your breath waiting for me to log 20,000 miles on any super oil. Possibly in Hawaii or Panama in a service vehicle....NO WAY in a normally used vehicle in a 4 season setting.
From what I remember reading back then and in posts since, a vehicle in the UK would never see temps cold enough for the difference between a 5W and a 10W to matter. Those blokes near Hudson's Bay and Narvik, Norway need to heed the recommendations closely.
Regarding, "Maybe the dealer was using up bulk quantities or lowering cost for customer"; you're much too forgiving. The guy had the 10W-40 on hand and just used it instead of ordering up the proper stuff. I suppose he should get a point for including the 10W-40 spec on the bills.
A counterman at a chain tire store here (Mavis) said they "throw a 10-30 into everything". Being cavalier with others' assets must tickle their fancies.
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