A friend of mine who owns several Brit. cars of various marques passed this onto to me, and I thought I would share with JH owners who frequent the Board. Go to the following site for an informative read: http://www.ttalk.info/Zddp.htm.
In light of the info presented in the above link, and since Valvoline VR1 is essentially no more expensive than the Castrol 20w-50 I have been using, I will make the switch, for both engine and gear box. PK