which engine oil viscosity?

The family has 4 automobiles-Ford Bronco, crown victoria, toyota sienna and saturn SL2; not to mention Son's 4 stroke dirtbikes. Upto now, I have 3 diff viscosities of motor oil- 5w30, 10w40,5w20 between the cars. My question is how important is it to stay with the manufacture recommended viscosity? I live in Georgia where the temps rarely get below freezing but does get up in the high 90s during the hot summers. Would it do any damage to engine to go with 10w40 in all the cars?( its kind of a hastle to buy and keep all those diff viscosities)

Sure, I think 10w-40 would be all right since it never gets very cold where you live. The 5w-30 and the 5w-20 are gas saving viscosities. You may get a little less miles per gallon in the Sienna and the SL-2. The heavier oil causes internal drag, but it would hardly be measurable. How about some input from out there. Would the heavier oil hurt his fuel mileage? What about wear and tear on the engines? Would it be better to just use 5w-30 in all his vehicles?