HI Hanne,
It would be helpful to have a few more details on what you are setting out to do.
- Application?
- Oil Type and Oil Volume?
- Current Oil Temp. and desired Oil temp.
- Ambient Temp.
We do have some small filter systems that can be customized to include heat.
http://klassenhydraulics.com/products/finefilter.html
Cheers,
-Perry
perry@klassenhydraulics.com