I may be totally old school, but here is a thought. Newer cars all have overdrive transmissions and are turning fairly low rpms (2000 or so) on the highway. An older car will be in the 3000 rpm range or so. I would think that oil is hotter in a higher revving V8, so a heavier weight is appropriate in warm weather. This is just an opinion, but manufacturers did not recommend 30 weights above 86F in the past. All that being said, I don't think you would have any problems with any quality 10W30. I use a 15W40 in the summer, expecting to see ambient temperatures in the 90-100F range.