I have an '84 300zx,na, which I purchased when it was 6 mos. old. I began using Castrol, because at the time it was advertised as a "high RPM Oil for Foreign Cars." Have always used 10w30 and on occasion have used 20w50 because of the Temps in AZ.
The bottom line is that I have 148,000 miles on the car and the car just got through emissions very much below all of the standards.
This is not an endorsent for Castrol, its only to say that I have used 10W30 in a hot climate and have had no problems after 20 years. (OH, let me restate that, no oil problems)