I have done the oil change now thanks, Royal purple seems hard to get hold of here, but does seem to have good reviews by the racing clubs for all their products.
But i opted for 75w140 fully synthetic by castrol, the highest weight oil specified in the owners manual.
There is still a slight axle pinion whine but the annoying juddering has gone. So well pleased with it. Cost £13 per litre, so 2 bottles of that and its had a good effect.
Now I use it every day its getting more use than before and no judder yet.
Also didn't put any additive in.
Cheers Jon