I'm moving on from the excellent Millers CSS 20w60 semi-synthetic to this at my next change:

http://classicoils.absolutewebhosting2.co.uk/Produ...

At £33.00 for a quality high zinc fully synthetic it's a steal, and the 15w60 weight is perfect for the Rover V8 too.

Have been looking for Millers CSS and eventually got talking to a Millers wholesaler. He said he would not recommend CSS 20/60 for my 4.0 as he said its for "old Healeys etc".
He recommended CFS 10/60 Full Synth at £60.00/5 litres.

I am thinking of going down the Valvoline VR1 route as I can get it locally. Just need to convince myself that is is sufficient for these engines.

Valvoline 20-50 vr1 is working well on my engine. The regular oil change is the key. I don't go over 5k miles and it can be bought in bulk at £30 for 5ltrs. (buy 20 litres a time)

High Zinc content in the oil is only REALLY important after a rebuild/crank/cam work etc during running-in.It helps the surfaces match to each other during that time - friction is greater and so there is much more heat - but once this time is past the benefits are marginal. It won't do any harm, though.

Edelbrock do a mineral oil specifically for this purpose - as do many others I am sure.

Good old 10/40 semi should then be fine - providing it is changed regularly