Chris,
I understand your concern, with very old engines that have an unknown history and the possibility of having been run on non-detergent oil, it is better to err on the safe side and only use a Straight non-detergent oil, until the time comes for an engine overhaul, when it will be internally cleaned. As you said it was a Leyland 690 ( I know the AEC 690, assume it is the same?), that dates from the 1960's so would assume it to have been run on modern detergent diesel oil.......but who knows?
A good way of telling, would be to drop the sump and if thick with sludge in the bottom, that would indicate having had a straight oil in it. In fact I would drop the sump if only to look for any signs of bearing metal and to check the bearings ( remove caps for a visual ).......at least it will give you peace of mind.