Oil in the radiator and in the radiator fluid reserve tank

I own a 2000 Nissan Xterra . I recently found oil in the radiator and in the radiator reserve tank . I am concerned that I may have a leaking head gasket or a blown headgasket. However , the regular symptoms of a blown head gasket are not there.
- The oil on the dipstick is like new . The oil was just changed about a month ago.
- I do not see any white smoke coming out of the tail pipe.
- The car does not overheat.
- The car runs great and does not show any signs of engine problems.
- There no smoke coming from the engine compartment.
I flushed the radiator last week. ( I was not able to remove the engine drain plug and it seems like the engine drain plug has never been removed before). I use new coolant to fill up the radiator and the fluid reserve tank.
I checked the radiator today and it has oil . The reserve tank also has oil in it. Somehow , oil is getting into the radiator but the usual signs of a blown head gasket are not there. Has anyone experienced this type of problems before ?. How can I find out for sure if it is the head gasket or if it is something else ? I need your assistance. A mechanic told me today that just because I have oil in the radiator and the reserve tank does not mean that I have a blown head gasket.
I love my X . It has 128k miles on it but it runs like new. I appreciate any assistance that I can get. Thanks

if you have an automatic transmission, the trans cooler is inside the bottom of the radiator and it may be mixing fluids. check your trans fluid.

Thank you.
Does it mean that I have to change the radiator or is it something that I can fix by just removing the radiator and checking the lines.
I appreciate your assistance.

Can I just remove the trans cooler inside the radiator and replace it by a new one ?

I doubt you would find ATF oil in the radiator. If that was the case, then you could drain the water into the transmission lines.

There are two separate radi cooler systems. I would think a leak would be oil out of the radiator but not mix with the water via Trans.

I would have to see you agreeing that fresh clean oil is gold and not red atf. So, pop the radiator cap off, start the car. If the radiator bubbles, then that is compression loss and it bubbles into the water jacket and not fire off with the kinetic it needs to help fire off fuel.

Take a radiator pressure check to determine a blown head gasket is my guess and not the Trans fluid.

if this is the case then you have to replace the complete radiator. in a case like this, with the engine running you have more pressure on the trans lines than in the radiator, thus pushing trans fluid into the coolant. with the engine hot, the cooling system has pressure on it, when you turn engine off and there is a leak in the cooler it will push coolant into the trans fluid. this is how the fluids mix. if your trans fluid has a pinkish tint to it, then this is your problem. a blown head gasket will usually give you a lot more symtoms than what you are describing.

Thank you everyone for our assistance.
I appreciate it.
Thanks