Oil found in coolant reservoir tank - 1997 Honda Civic

-yes--but keep an eye on the temp gauge
if you have had the vehicle from new then it could be a oil weep from a gasket that is getting to the coolant system
It could also indicate a failing transmission oil cooler that is in the radiator tank
If the vehicle is second hand , it could be the residue of a previous repair that has slowly found its
's way to the reservoir
Perhaps a compression test to determine if it is a head gasket and the a professional flush and refill with the correct coolant additive is in order

A few things could have happen,
1 you overheated and blew the head gasket and now the oil in mixing with the anti-freeze,
OR due to high Mileage the head gasket has just failed and is leaking into the anti-freeze journals inside the engine.,

Did you check the oil to make sure it's not a white'ish sludge on the inside of the cap, you might be mixing both back and forth..

1. Check the hoses and clamps going from the reservoir to the radiator and from the radiator to the engine. Make sure they are flexible and in good condition.
2. Check the reservoir and radiator for leaks.
3. Replace any parts they may be failing. If no leaks are found proceed to next step.
4. Check engine oil. If oil has coolant in it you may have a blown head gasket or crack in the head or engine block. Another sign of a coolant leak into the cylinders is white smoke coming from the exhaust after the engine is warmed up.

I was going to say that white smoke wouldn't be oil but I see that you figured that out. Look for loose coolant hose clamps on or near the reservoir or possibly a small crack in the reservoir itself. As a general rule for gasoline engines: White smoke = coolant, Blue smoke = oil, Gray smoke = fuel, Black smoke = "Uh oh, that can't be good." :-)

leaking head gasket

If your oil and coolant mix it means your head gasket is blown. If it is just an orange or beige paste that you are seeing in the coolant reservoir or radiator thats not oil.

Yes you can do it yourself

Degas Bottle Removal

1. Drain radiator (8005) until engine coolant is out of degas bottle. Disconnect radiator overflow hose (8075) and engine coolant vent hose at degas bottle.
   

1. Remove power steering oil reservoir retaining screws and position power steering oil reservoir (3A697) out of the way.
   

1. Remove degas bottle return hose and clamp. Remove degas bottle retainers and remove degas bottle.
   

Installation

1. Position degas bottle in vehicle and install retaining screw and nut. Tighten retaining bolt and nut to 9-12 Nm (80-106 lb-in).
   

1. Connect degas bottle hose and clamp to degas bottle.
   

1. Position power steering oil reservoir onto degas bottle and install retaining screws securely.
   

1. Connect radiator overflow hose and engine coolant vent hose to degas bottle.
   

1. Fill and bleed engine cooling system as described. Check for coolant leaks and proper engine coolant level after engine reaches normal operating temperature.
   
   
   
   
   
   Item Part Number Description 1 8100 Radiator Cap 2 - Nut (1 Req'd) 3 - Bolt and Retainer (1 Req'd) 4 - Nut (1 Req'd) 5 - Bolt (1 Req'd) 6 - Coolant Recovery Hose 7 - Coolant Return Hose 8 - Coolant Recovery Hose 9 8A808 Degas Bottle A - Tighten to 9-12 Nm (80-106 Lb-In)

Hope this helps.

Zeppelinx12

replace the oil cooler first before you change the head gasket ,but normally a 10 year old vehicle often has gasket fatigue ,not made like a good old fashioned copper head gasket ,they rarely went wrong

either a leaky head gasket or someone put oil in the overflow, it happens, clean it out drain the coolant from the entire system and refill with new long life antifreeze, also add a can of good radiator sealer.

Twice in two years now my Saab 9-5 has had a pin-hole in the oil cooler, which has caused oil to flow into the reservoir. A way to tell is if coolant/oil is coming out of the reservoir cap. The reservoir cap is rated for the pressure of the coolant system, which is significantly lower than the oil pressure. When there is a hole in the oil cooler, oil will be forced out at relatively high pressures, causing the reservoir to fill and then over pressurize and spill out of the cap.