Oil in coolant resevoir after coolant change

If it hasn't gotten worse, it was most likely a hose that started breaking down on the inside & may have been the one you replaced. You could drain & clean the tank, then keep an eye on it going forward to see if it persists. Check for additional spongey coolant lines.

 

If it has a coolant - transmission cooler check the transmission fluid. That can cost you a transmission.

 

If it has been parked crack the drain plug. Coolant will settle to the bottom. On the fleet truck that happened with here fluids went both ways. No one said anything it was time for service. Trans fluid was nasty looking and changed it, the cooler and flushed cooling system. Worst stuff I have ever seen to get out of a cooling system. Trans went out out of state a few weeks later. That one was really bad but sounds like yours is not. Just check it.

 

I don’t suspect oil in the coolant from your photo. If there was there would be an even layer on top of the coolant.

Some ISX do not have a coolant filter option. What I suspect is you have sediment that has been stirred up and is stuck to the inside of the surge tank.

My plan of attack would be to flush the system with cascade and water multiple times. You’ll most likely have to replace the surge tank to get rid of the staining if it really bothers you. Or just leave it alone.

 

It's black rubber from a hose. When it's cool, stick your finger in the reservoir and give it a wipe. Bet 10 cents it's particulate and feels a little gritty. I get that every time I have to drain coolant to work on the engine. If it's not oily, just drive the truck. IMHO

 

Paccar runs all silicone hoses, no rubber.

Unless they were replaced……