Either will be fine, If it's a high mileage engine I'd edge towards the 15/40 but assuming the engine is healthy it really doesn't matter.

As for the GTX, no Castrol oils are true synthetics (take a look at the rather poor cold pour points for proof) - they are a marketing over contents brand, you'd be wasting your money. Get yourself down to "The Range" and buy 5 litres of Unipart semi synth for £13.99 or fully synth for £17.99, probably better than the Castrol stuff and half the price.