I'm not sure what kind of oil Their Oil really is. If you really want to aggravate the dealer then ask him for the results from independent studies using numerous real engines that proves his statement. I'll bet money he either has nothing or just the usual sales literature from a small miracle oil company. Just refer to the owners guide for proper weight and API rating and buy oil from one of the big name brands like Mobil, Shell, etc.

'Air-cooled service' is a higher spec oil than 'Water cooled', generally. I cannot tell you exactly what the differences are, but it is sufficient for my purposes that the classification exists. I use only 'Air-cooled service' oil in my lawn mowers. Mr. Roadbike probably uses air-cooled service oil in his Harley. My mower didn't cost $20,000, but I'll spend $5 a year more for the top spec oil for it.
Well, I see I've been working on the assumption that you would automatically know why there is a higher spec for air-cooled engines, which is really your question. My bad. Engine heat in air-cooled applications is not regulated well at all compared to water-cooled engines. Heat can spike quickly and there can be hot-spots in the cylinder. A heavier-duty oil is designed for these applications to avoid premature engine failure. Will the average mower do fine on automobile oil? Most likely it will. Is it worth it to spend a few bucks more a year for the very best? Whay not? Makes me feel a lot better. There are two oils that in my view are the very best; Opti-4 10W30 or SAE30 and Royal Purple 'Max-Cycle' 10W30. I have been using the RP 'Max-Cycle' in one mower for two years and I have a 20-year history with Opti-4 30W, which I currently use in my #2 mower. The mower I started using it in in 1990 is still in service at my brother's house and it has had no internal engine repairs, not even a lower seal. That engine is a B&S 3.5 HP, non-commercial.

This is an easily solved problem but the discussion is being muddied by a lot of useless information. As I said before: Just refer to the owners guide for proper weight and API rating and buy oil from one of the big name brands like Mobil, Shell, etc.

Lawn mower and engine manufacturers have a distinct advantage because they employ numerous well trained designers and engineers to build mowers and make informed recommendations for their maintenance. You will find that some of us feel better when they use some off-brand oil, but that good sensation is grounded in hyped advertising and not experience.

Briggs and Stratton recommends 5w-30 Synthetic (ILSAC GP-2, API,SJ/CF) in all their engines at all temperatures. See the link below.

Here is a link that might be useful: Briggs and Stratton

As Rdaystrom advises 5w-30 for lawnmowers . AND 25w-50 for Gen Sets and Harleys lol . I use to run straight Esso Avation 105 (Exxon for you South of the Border) in my Harleys till the New Syn's came out . A lot more Demon Heat from Air Cooled Engines with extreme Temperature swings vs Liquid Cooled Equipment . I prefer Amsoil but as RB advises any Quality Oil from the Big 4 will suffice within the proper Viscosity Rating .

I stated 10W30 and SAE 30 because those are the only oils that have been specified by the engine manufacturers I have used (Honda and Tecumseh) in the last ten years. It's interesting that B&S now specs the 5W30 Syn.

Ckeck out this site it may answer a few questions
http://micapeak.com/info/oiled.html

Sax: The recommendation for change has been just within the last few yrs. My New Yamaha and older Honda (Gen Sets) are both running on 25W-50 Syn now . The Honda is 1987 and was originally spec'd for Sae-30 . The Yamaha only 9 mths old came with multi grade syn oil spec'd from the get go . As for Royal Purple you cannot get much better than that , used it for over 5 yrs running Open-Wheeled Modified . The Opti I used many yrs ago for chainsaw usage and was not dissatified in the least . The only reservations I have ever had was with marine grade 2-stroke oil being used for air-cooled units . However I have since been enlightend within this assumption on Lawn-boy Mower's by our forum expert :) .

Dirt modifieds? You ever been to VMS in Saluda Va? 1/2 mile dirt. I also like Williams Grove in Harrisburg PA when they have the 410 Sprints. Nothing like an aluminum block 410 C.I.D. running Nitro in a 1000 lb Sprint car.
And who is this 'expert'? My dad used to say, 'An 'expert' is a has-been drip' Ex-spurt?

Around these parts, an 'expert' is a guy in a white
shop coat, ten miles from home.

Yep have seen the Williams Grove Race Site . Have attended Eldora and Sandusky Tracks . Usually ran 1/3 and 1/2 mile asphalt with a non winged modified 1980 era 360 Mopar Magnum running AV100ll Fuel . The largest track I ran was Cayuga 5/8 Mile located in Southern Ontario . Ran Royal Purple religously Engine and Gear lube's. Ex-spurt.. Roflmbo ! Heck getting old I think it was Orange he has a proprietary blend of TCW-3 and MMO and swears by it , with no carbon related hot spots...etc. anyhow he has ran LB for yrs and knows his 2-stroke mowers lol .