Suggestions needed for oil in a 400 and a 365 Boxer

It is time to change the oil and filters on my 1977 400 Auto and my 1976 365 Boxer. In the past I have used Castrol GTX 20w50 but I can't seem to find this in the UK at the moment, only GTX 10w40. I am reluctant to use synthetic, such as Mobil 1, as I don't want to create oil leaks. I have seen Shell Helix 10w40 locally at a good price. Should I use GTX or Helix or something else?

I will be using UFI filters.

Simon

I would wait and try to find the castrol. I sure as hell would not put synthetic in a Boxer.

Simon,

I've run Mobil 1, 15W-50 in my 365BB for at least 15 years without any excessive leaks. In the same breath, I've used 20-50GTX in virtually every other car I've owned...I, too, have heard the "rummors" that a synthetic may leak more. I've never found that to be true but can understand the engineering on how it could leak more...

A 10W-40, dino-based oil would be a safe choice, as well...presuming reasonable operating conditions.

Regards,
David