4K High oil temps and cooler question

[DK]

Hi all,
my '87 4k oil gets pretty hot on the highway, usually hitting 130C
(266F) going 70-75 within several minutes of driving, sometimes more
if the weather is hot. Once getting off the highway, it drops within
minutes down to the 90s. In town driving I have not seen more then a
100c on a hot day. This makes me "believe" that the gauge is fairly
correct.
Coolant gauge is usually between 1/2 and 3/4 and cooling fans come on
as needed. I am running 10w30 Castrol GTX oil. Minimal oil consumption
between oil changes.

Should I try a heavier or different oil. I recently got a used oil
cooler of a NF/NG engine but have not installed it yet, hoping I will
be able to lower the temps. I have not yet installed it, as I am not
quite sure how to mount it securely.


TIA,

Dave