A lighter weight oil gives less protection than the heaver ones.  The main reason the auto companys say to use 5-30 is for the gas milage.  The lighter oil is easyer to move parts through.
In everyday use this is just fine.  When I get higher miles on my vehicles I go up to higher weights.
Mine, with 154,000miles and the way I drive it I run Mobile 1, 15w-40. ÂÂ
I've never had trouble starting it, I get about 20 miles to the gallon (32's with 5.83 R&P).  I've run it all day long on the sand dunes (you'll not find anything harder on an engine than pure sand) with the peddle to the floor and she just keeps going and going....