http://motorcycleinfo.calsci.com/Consumables.html#Oil

I really like this authors list of recommended oils! My personal preference is Maxima Ultra for my ATV's (I prefer the Group V Ester Synthetics), Mobil 1 SUV for my truck, and Rotella T Synthetic for my old beater van.

 

10w40 in the cars and in the trike.

The author states that 10w40 should not be used in engines that use the engine oil to lubricate the transmission. However, if you look at the other article, it states: "Based on this result and the VI numbers above, it would seem that 10w-40, 5w-20, and 5w-30 oils cannot be safely used in motorcycles for more than 1,000 to 1,500 miles."

I think it's safe to use 10w40 in your ATV. If you have your oil in for 1,000-1,500 miles, you probably already have bigger problems than a viscosity breakdown.

 

I've been using Mobil 1 MX4T. It's designed for wet clutches. This same guy also choose Mobil 1 oil filters as one of the best.
About $7 a quart at autozone.

 

Yet several of the oils he recommends are 10W-40!

It requires a lot of VII's to make 10W-40, and that is why he is not recommending them, and he is also coming from a street bike perspective. I personally have no problem with running a 10W-40. See http://motorcycleinfo.calsci.com/Oils1.html