Air cooled mower engines run HOT, much hotter than water cooled engines. If you use multiviscosity oil, they burn it pretty fast.
Most of them are splash lubed, too, so thin oil won't lubricate the crank very well.
I've been using 15W50 Mobil 1 in everything this year -- needed to fill the mowers (they are getting a bit long in the tooth and using oil) and only had the Mobil 1 around, used all the 30 st last year. So far I have less consumption, but will see at the end of the year.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!