The numbers are viscosity (how fluid the liquid is: water being about 1w and honey being about 75w).
Starting a cold engine in Winter: you want the oil be be thin (5w, 10w), but once the engine is warm and working, you want a thicker oil so it will stick to the cylinder walls to lubricate the engine. In high engine heat the thinner oils just flow off too quick and the engine will burn, that's why we have the dual number - combinations oils available in the Winter.
For summer, the 30w oil is great, and even down to 40-50ºF it will work great. If the tractors are outdoors and the temperatures are getting down to 10-20ºF, you want 10w30 oils. Having a heater on it, or in the garage with a light bulb or pad heat on the engine in the Winter makes the block and oil actually more like 40ºF, so then 30w is still fine. If you're running a larger tractor doing heavy work for 8 hours all day, especially on hot days, then you may want a lot heavier oil, but heavy oil takes more effort to turn the cylinders over when starting, until the engine is warm.
So, what you choose to use depends on your use, when you're using it and storage. Just one oil is nice and then you can buy it in larger containers to save money. What brand to use, has lots of different opinions. Then there's the synethic oils.
I like life simple & my tractors are all garaged (unheated, but still a +temp gain); and I use small bulbs on the really cold Winter days here in Michigan (between the battery & block, under a blanketed tractor, for pushing and for throwing snow, two tractors), and just 10w30 oil all year.
Edited by GlenPettit, October 31, 2014 - 05:24 PM.