What does it mean when oil is in the overflow tank

Overflow tank for coolant

well the tank must never be full, when engine is cold
was it? (read operators guide to see methods)
water expands hot, and the tank captures this expansion
that is normal.
what does "until its all gone", mean, what is IT?
there are lots of tests.
the first one, is,
IS the engine overheating,?
try a new rad cap, they are dirt cheap.
and, is the oil or carbon in the RAD neck, cap off , cold engine>?
is there water in the engine oil, milky dip stick end?
same with A/T trans? milk, or pink/red ATF.
is there steam going out the tip of the exhaust pipe on a fully hot engine>? yes/no?
is air and water exploding in to the side expansion tank.?

cold engine
remove rad cap'
fill rad with 50% AF.
new cap back on.
put 50% AF 1/2 way up in to expansion tank(its marked clearly)
start engine , does it overflow now>?

THIS MEANS THERE IS A MIXTURE OF WATER AND OIL SOMEWERE,CHECK THE WATER PUMP GASKET

Head gasket or cracked blocked

Could be tranny fluid or engine oil. Check trans and engine oil and see if they have water in them.Water in engine oil can mean a bad head gasket but could also mean bad intake gaskets or oil cooler if it has one.( rare on this car)Water in the trans most of the time means oil cooler in the radiator. This is bad and will mean transmission failure soon. The chemical reaction of the antifreeze and transmission fluid damages clutches and internal seals.

If no leak is found inspect the inside oil fill cap to the engine. If a yellowish cluster of pudding I call it is stuck inside the cap that is caused by coolant mixing with the engine oil. That means your head gaskets are at fault and they are in need of a replacement

Head gasket or cracked head

Yes, these days when someone says radiator cap they mean the cap on the coolant overflow tank.
Hope this was helpful.

Yes, oil in the antifreeze typically comes from a blown head gasket. To verify, start the truck and watch the overflow tank you should see what looks like a back flow or bubbles. By the time the truck get to normal temp, it'll be obvious.

If you mean the coolant overflow I would check the oil dipstick for water oil. It will appear white/tan. If this is the case I would firmly suspect a head gasket.