As  engines  get older,  or are 'well up in hours'..   I like to  change the oil a little  quicker than specified..or  at  worst,   don't let it  go over!   Worn rings  let more  contaminants   past,   and oil  is  contaminated  quicker.   What the engine  just  completed  too is  a consideration.   One  that just finished up   field work  is  a good time to   change  before you put the baler on.
 Get the dirt out!!!   
I  run  15w40  year round..    and  never worry about   if it's  10 below or  110 above.    As far as  brand,    I run   Mobil  fleet.     It's a conventional  oil.    For me,  it's  handy,  and  the best price on that class of  oil in my area.  
My thoughts  are,     regardless of    oil,   contaminants  must be  removed.   Running in the  dirt,   tired  rings,   whatever,   is  going to compromise  a  150  hour oil just as quick as a  500 hour oil.