Looking at a 92 gmc suburban .engine never gets warm found what I think is 90wt oil in the radiator.Owner admits to putting no-leak in .No water residue in engine oil. Any ideas?

looking at a 92 gmc suburban .engine never gets warm found what I think is 90wt oil in the radiator.Owner admits to putting no-leak in .No water residue in engine oil. any ideas?
The "90wt" may be stop-leak gelling. Are you having any cooling system concerns?
No heat and/or engine overheating?

First, inspect for any evidence of previous leakage. Check the condition of the hoses.

You will likely need to replace the thermostat and flush the entire cooling system and retest for leaks. Pay particular attention to areas like the water pump, intake manifold etc.
I hope this helps.