Why does everyone insist on referring to the owners manual as Gospel?
We discussed this a few weeks back and while I generally agree with TR6BILL, I need to remind folks that "The Law" according to Triumph was written into the owner's manual well over 30 years ago, eons before many significant advances in modern oil technology. 20W-50 is an "excellant choice for high temperatures, heavy load, and high revving engines." Plus, "Castrol GTX is engineered to provide maximum protection against BOTH viscosity and thermal breakdown." Read between the lines here; I don't think TR's run particularly hot as cars go, nor do I think they undergo great load. So I don't feel heat or load are the greatest issues when choosing an oil for these cars (unless yours is a daily driver towing a trailer in the southern US) Yes, 20W-50 has great viscosity endurance, and for Bill in Louisiana that's fine, but it's also considerably thicker in cold weather startup when, as we all know, engines are prone excessive wear due to lack of oil pressure. That, I feel, is the more pressing issue here.
In conclusion, you can swear by the ancient dead sea scrolls found in your glovebox, or take note in what's going on out there with the new, super premium SJ class of oils. For many TR's out there sitting in their cold northern garages during the winter, the monthly startup is far more critical than all of this talk about heat, RPMs, and load. Depending on climate, for the northern half of the US, I say go with either a 10W-40 or, dare I say it, 10W-30. As for Colin, he lives in Mass., so I think this is particularly important.
By the way Colin, of equal importance, if you haven't already done it, replace the canister filter with a new spin on. Use Fram filter PH-3600. Good luck.