A dark brown color inside of the oil fill neck (walls)? J30!

[FarFetched]

There is no sludge, just brown color inside as far as I could see. Does it mean that engine has been overheating, or just poor maintainance? Also, on a hot start RPMs slightly fluctuates and then stabilzes (around 1500RPM). Feels like RPMs want to go up, but something is holding them up! Any input? Also J30 had an emmission failure last year, had been repaired. Any ideas what is the first thing to blame? Is it common to hear en exhaust leak on J30?Thanks!

[driverdriver]

infrequent oil changes with cheap oil.