Oil for E23 728

The engine has over 300. 000 km and I hear some ticking an can se some smoke when high RPM. I have changed the sparkplugs and the old ones was not good. I have also put some injectorcleaner in the gastank, that did help. Now for the oil, I thinking of using Valvoline 15-40 Racing oil, any one with some tips on that?? And I am going to use some oilcleaning fluid that I put directly in the oiltank and let the motor run for some time then change the oil and filter. Have some one tryed that?? And how many litres of oil do I need for a oil change with filter??

Ps. You can answer in German