Since my local Indy closed, I’ve started doing some engine maintenance. First job was to change the oil. The car has had routine oil changes at 4,000 miles
After draining the sump, the filter was emptied into a seperate jar. I was surprised at the thick mucky oil st the bottom of the filter.
That got me thinking about the remote under-trunk oil cooler; should it be removed and cleaned?

Jim it depends on how long its been on and if your temps have come up,oil coolers are very good @ building up oil varnish which can reduce heat transfer,presuming this is your only cooler ?.
If any of the above is not up to scratch it could benefit from an ultrasonic clean,you will be good.

Sounds an awful lot like condensation and not getting oil hot enough.

What do your oil temps run? Despite the old wives tales 180F is not hot enough to drive out moisture properly especially on short trips and infrequently used motors.