According to XOM and SOPUS, thirty grades are fine for this engine.
Interestingly, Mobil recommends only M1 or MSS 10w30s and AFE, no 5w30s, with Pennzoil recommending only PP or Gold 10w30s, or Ultra Euro 5W-40.
Not sure why neither recommends any 5w30s.
Valvoline recommends only Synpower, with no grade specified.
Remember that the inline six used in this car is a relatively modern design, having nothing in common with either the old V-12 or the ancient twin cam six.
Now, in Cleveland, this car will have a fairly short salt-free season, maybe six months each year.
How much will you drive it over the season?
You could probably get by with a change of either Mobil 1 High Mileage or Mobil 1 Extended Performance 10w30 or Plat 10w30 once each year in the spring.
It really shouldn't be necessary to use any really exotic oil in this engine, nor should it be necessary to change it more often than every 5K.
Incidentally, you should bring this car to Mid-Ohio for the vintage event next weekend.
You could park in the infield with the other Jags, and if you like intersting older cars, you'd really enjoy the event.
The vintage racing is awesome as well.
We'll be there in our old BMW.