"my new forged engine is using about 1 to 1.5L of oil per 2500km"
+1 for leak down test.
+ my 2cents
Check for wet oil on plugs to see if your oil is being pushed up/sitting on top of your pistons,
Drain oil to see if there is fuel in your oil pan.
Consider not driving it, if you can, until you work it out as it can lead to another re-build..