Replaced original thermostat with an OEM Toyota one. Also did a drain and refill of radiator with Toyota sslc coolant and washed overflow tank with distill water. Went to check coolant level today, found sliver color residue in coolant. Silver color is more obvious when put on paper, marked by green circle in picture below.
What is this residue? Where does it come from? coolant, thermostat, or engine?
The truck is a 04 with has 118k miles, drives and cools normally. Original owner. No leaks. I also did a block test, color looks okay, blue. Regular coolant change every 30k or 3 years.
The coolant change used half gallon of sslc from a jug with recent pink cap (opened a year or so ago), rest is from a sealed sslc jug of older yellow cap packaging from a few years ago. Didn't see silver residue in either jug when pouring.
Any comments and suggestions are appreciated.