Old wives tale, plus it's a British engine. The only time it's NOT going to leak oil is when there is no oil left in it. The worst modern oil today is going to be better than anything available when it was new. The cheapest oil you can find is just fine. If it was mine, Mobil 1 5W30 since I must have over 50 jugs at 10 cents a quart.
In his case I think it would be wise to stay away from synthetics even though most conventional oils are pretty much synthetic blends these days. In a old engine with those kinds of seals and such , changing to M1 might bring on a lot of troubles.