Oil cooler question

[68 Tango]

Soooooo it's summer now and I bought my PA28 140 in December and have flown several cross country trips to Vegas @ 75% or sometimes full throttle at 7500 & 6500ft and my oil temp stayed consistently in the green, Ok I know that was winter and Spring and now it's summer , but I've only been able run at about 60% or my oil temp goes in the red, I enrich the mixture and no temp change, I reduce throttle to about 2000rpm and the oil begins to cool, is this because of summer heat (Los Angeles has been 90+f for a couple of weeks) or could my cooler need work?
Thanks

 

[JimC]

 

[dkallen]

 

[vaflier]

 

[Ed Dartford]

 

[baron_beeza]

 

[TEG916]

 

[Domenick]

 

[magman]

 

[DaveB]