What is wrong w/ HIH. It should warm the oil better than HOH.

BTW- I have a home made system with the following:
Tank w/ home made Heat exchanger
HIH
12V aux pump /pre filter
6 port Pollak valve
electric heated filter
FPHE
Vegatherm

I have a gauge just before IP. Normal oil temps are 180-200F. I recently added the FPHE to help even out temps in the NewEngland winter. I have been running this system for over 1 yr/25K with little or no issues. I had a similar set up on a prior vehicle for about the same amount of time. Both prefilter and main filter are the std MB types for simplicity(less parts to carry).