High Mileage Oil or oil to SF (From SM)
First to answer your question--The syn mix will help; and Castrol is Top Shelf--Full syn might be even better--depends on the motor.
These High Mileage oils help with leaking seals--if you have no seal leaks. I'd still go for a syn blend or full syn and add a ZDDP Concentrate--or buy it full synthetic with ZDDP already in the oil (to 1965 standards or SF). If you go HPS Royal Purple it's already there and you don't need an additive which is at best--questionable; I use HPS at 10w-40 after installing aftermarket heads, cam chain, and intake--on a block with 120,000 miles and drove 200,000 more--hard miles too--I mean running it mean--an old 65 A-code I had rebuilt 5 years before and drove daily then did the "boost kit by Edelbrock".
High Mileage oil is for seal leaks--if you have a good running motor and no leaks--the full Syn will extend your engine life--ask around. I've owned 6 1965 mustangs two K-codes, three A-codes , and a T-code (6 cyl). I still have 2 (the best of the lot left), an A and K--With the ZDDP additive (which is a must on every change for cars built before 1990)--do the research on oil differences now and back in 1965 SM vs SF. The other answers you got were good points both... but ask yourself... why is there a high mileage oil and why the synthetic? why do cars go 300,000 miles today with else but a tune ups? Why do my cars run for 300,000 miles since the oil technology has improved? After your car is well broke in--say 5,000 miles... why is it okay to use full synthetic now and why? There you go--I'm an expert on oils... chances are you have taken great care of your car and want the best for it--ask around (watch out for old guys like me who are uneducated though on the new technology of motor oil.) I still like to study and I still like driving my mustangs hard!