Jake,
A quick Google search revealed these: 
http://www.reiffpreheat.com/Article-Visser3.htm 
http://www.elephantracing.com/techtopic/oiltemperature.htm

As to be expected there is a large difference of opion out there but 180 to 210 seems to be ideal. It just so happen I have a Blackstone Labrotories http://www.blackstone-labs.com/index.html oil test kit sitting around.I will try to send off a sample at the next oil change.
Thanks,
Ken Lewis
http://neksiwel.20m.com/
in statu viae

  ----- Original Message ----- 
  From: Jake de Villiers 
  To: Ken Lewis 
  Cc: vanagon@gerry.vanagon.com 
  Sent: Friday, June 05, 2009 10:50 PM
  Subject: Re: Oil cooler installation pictures...

  180 degrees F is too low Ken - you won't be boiling off the condensate that's coalesced in your oil sump.  You'd be much better off at ~210 F