I found oil in the water reservoir.The car is a polo GTI manual 2007 model. Temperature of the car is normal.
First of all, clear that oil out and see if it comes back. It's not unheard of for oil to be inadvertently added to the coolant overflow tank (which I assume you are calling the "water reservoir").
If it comes back then you have an oil leak in the engine from the oil system to the cooling system. Several possible causes:
- Leaking cylinder head gasket
- Cracked engine block
- Warped cylinder head
Any of these is a serious problem and should be dealt with promptly. The most common issue is the leaking head gasket. If you're moderately handy from a DIY perspective, you can probably change it yourself. The gasket itself is inexpensive but it takes a fair amount of labor to get to it. Otherwise, take it to a shop.