Check what oil you are buying.
"Castrol GTX High Mileage" is a 15W40, so I wouldn't use that at all.
I'm just going by the info from Castrol and they only list the one labelled as "High Mileage" as a 15W40. see hereNot all of it. You can get it as 10W40. I have used it in my 156 in the past with no problems.
Dunno' about that? The Yanks were smart with their lubrication oils and additives. Remembering of course the demo' with Wynns Friction Proofing. Anyway, Valvoline claim their oil additives help keep old lipped seals flexible, and as it was very competitively priced, I used it, no seal problems, and whilst it may or may not have, I found it suited me fine.:
Beware a lot of these products. These friction modifiers can be very corrosive.
The trouble with the additives or oils, I forget which base group, is that they soften the seals because they make them swell. I don't know if it is a problem, but I imagine it may work only for a while, or even accelerate the degredation of good seals.