In addition to all the other cooling troubleshooting tips,  Is the EGR system
connected and working.  This makes a big difference in temps while hwy
cruising.  NOX emissions reduced by lowering of "Peak Flame Temparature".
 Lower PFT will lower head and piston temps,  which will reduce oil temp
also.  Even at 250 oil temp, should be able to hold 10psi oil pressur.  Note
that stock sensors do not turn on light until pressure down to 3-6psi.  If
engine was rebuilt, case probably not align bored causing leakage between
case and bearings.  This will reduce pressure also.  
Hope this helps,
Dennis.