I have the HD factory oil cooler on my 03 Fatboy. I found that it lowered oil temps. in the oil tank about 20-30 degrees, depending on air flow through the cooler. On the TC Softtails, the cooler is mounted right in front between the frame downtubes. This gives good airflow to the cooler but kind of blocks air to the front cylinder. When I modified the engine, the oil temps went up over 20 degrees from when it was stock with the cooler. This is one of the reasons I use Redline 20-50 in the cooler months and 20-60 Redline in the summer. Last week, in over 100 degree heat, I rode about 100mi on the freeway at over 80mph to meet some friends for a BBQ. When I got there, the oil tank temp was 215 degrees, which is totally acceptable.
I think the factory HD oil cooler is made by Jagg. Their version mounts the cooler on the outside of the L/side frame downtube, away from the front cylinder.