I would drop to W40. You should not be concerned with LL01. Yes, Castrol 0W40 will do fine, but as far as I know M62 was anyway recommended for LL98. Issue with W50 is that it will increase oil temperature. I would go with W40, 5W40, 10W40 (Castrol Edge 10W40 as far as I know is available in Wal Mart and is synthetic). Maybe high mileage oil would be better to keep seals flexible.