I run Brad Penn 5w-30 in my 340 in temps from 45-95 degrees.
I have tried 15w-40 but it was too thick and it made the lifters tick at WOT and on cold startup. Then I tried 10w-30 and the WOT lifter tick was gone but then I still had some minor lifter ticking at cold startup. So then I tried the 5w-30 and I have been running it ever since with no lifter ticking problems except for the at the very first startup after it has been sitting all Winter long.