I run 5w20 in my 250, year round, but don't drive every day. I have seen the effects of thick oil on oil pump drives and cam gears.
I am going to have to disagree with the meter holes mentioned above. 10w30 will flow through a meter hole at low temps like 10 w oil, this is true, but at higher temps, it does thicken up to 30w oil, what it does is lubricate like a 30w oil due to it chemical make up. The viscosity always stays the same.
Joe
Close but not quite right right. It doesn't thicken up, but doesn't get as thin as temperature increases. There's no oil that exists that has the same viscosity cold versus hot. Even water doesn't have that physical property.
Here's a good reference for oil related stuff. This link is where he talks about multi-viscosity rated oils.
http://bobistheoilguy.com/motor-oil-102/
