I use Valvoline 25W60 Race oil (Thats what is written on the bottle) on our VQ and Toyota and we change it every 750-1000km and comes out sweet. Used to use Torco and that was good too but too expensive.
Les, I strongly suspect you're chasing the symptom not the cause. Black oil can be either the oil collecting carbon deposits (which is unlikely in a well designed engine like the VQ) or more likely you are burning the oil through overtemp. Do you have an external oil cooler and do you log/view the oil temps (they can be quite radically different from the water temps in the VQ). Oil pressure drops can be a resultant of either oil breakdown (overtemp) or sheer. Either way the oil has broken it's longer chains and the lubrication properties have dropped dramatically. As an aside. I also use Mobil 1 which does seem to cater to overtemp issues better than other oils I've used in the past. (note it's an emotive evaluation).
As redmist said I am thinking you have overheating issues, I have a heat exchange on my engine could be why my oil looks clean, I've not had a oil sample checked but it looks good when it comes out.
