nigel1977, i used the slick 50 diesel oil treatment without any major difference. i used it @ 85000kms.
if you use the castrol syntec blend oil, you could even go up to 7000kms without an oil change!
i used to do this when i used to make a UWI run everyday from south... wasn't economical to change oil every month/5000kms or so.
refer to the diesel engine maintenance thread...