I found this Technical Service Bulletin on the net which applies to the mgb issued April 1975. It was on an American website at http://www.chicagolandmgclub.com/techtips/mgb/tsb_mgb.html
It may apply to American spec cars only but does not say.
Would be interesting to know if anyone has tried it and the benefits.
Think for the time being will carry on using the 20-50 engine oil.
updated 22nd Jan 2020
have been advised by a response from the MG Exp forum that a later bulletin retracted this info – that only 20-50 should be used.