Sludge in the oil filter

I had to replace the radiator and the heater core is plugged ... No major problems till now ... I have been loosing coolant here recently (not real bad) but cannot find a leak anywhere ... The truck doesn't overheat and runs a normal 195 temp ... Changed the oil today and there was sludge in the filter ... Never seen this before ... Any ideas ... Truck has 173,000 miles on it

What engine? It could be an intake leak allowing coolant into the oil. It would be good to repair that quickly, since coolant is not very kind to the alloys in bearing shells.

5.3l ... The oil didn't look milky when I changed it though only sludge in the filter ... And you're saying it's possible for it not to run hot with the intake leaking?

The transmission cooler lines run through the radiator check transmission dipstick and look at fluid and level. Get a loaner radiator pressure tester when running pump up to 10 psi watch gauge if pulsating you have a leaking head gasket or crack. Sometimes a water pump will leak then stop look at weep hole for deposits. Check drain hose for A-C and heater core if it has deposits maybe the heater core is leaking.
NAPA sells a combustion gas analyzer kit. As Bowtie stated Anti freeze leakage in engine means a rebuilt engine if not corrected.

Does the truck get routine oil changes at regular intervals? (3-4,000 miles)?
Do you use good quality oil? Does the truck see any short drive use, like 5-10 miles or less, (where it does not get up to full operating temperature)?

Early in my truck's life, I was changing it every 2500-3,000 miles. (Regular Valvoline "dinosaur" oil). When I switched to their Syn-Power synthetic, (around 100,000 miles), I kept up that interval for a while, then began extending it a bit, have been going 3500-4,000 miles the past couple years. I think the higher quality oil stays cleaner, longer. It now has 279,000 miles, and I have never seen even a light coating of sludge in the filter.
Let us know what you find out, sounds like it could have a gasket issue.

short runs and conventional oils is the cause most times ... winter temps will do this with the NON synthetic oil ..

these engines do require synthetic if you plan on keeping it ..

any coolant loss in the engine leaking is easy to figure out . pressure test >........

you never stated the oil type so it is the dino oil IMO..

I agree with j_cAT. Conventional (non-synthetic) oil, and short trips in cool/cold weather will cause sludge to develop.
My truck did have a very slow drop in coolant level, (like less than a 1 inch drop in the coolant reservoir), over a 6 moth period. Never saw any evidence of leaking on the ground. Figured maybe I was losing a head gasket. Eventually I did see a few drips of water while it was idling. Water pump was the culprit. Topped it off, and waited a couple more weeks, by then it was needing some water to be added. Replaced the W.P. and has not used a drop since then, (over a year ago).