I dont bother with the oil in the cooler, but it doesnt go black, it only ends up slightly darker then new oil, I think the cooler holds around 1.5ltrs and when I drain the sump I put in around 5 or 6 litres depending on if its before a track day or not, so its getting 80ish% of fresh oil and a new filter on a change.
My oil does smell like hydrocarbons when its getting dark and ready for a change but Im told that higher boost does this.
Peace Love and Mungbeans.
On a lighter note I put the car in for a tune and have to replace my wastegate and exhaust manifold as they are both fubar