Oil cooler can be mounted anyway up you want they are self bleeding and as such will force air out of the system however they are mounted.

It won't cause a problem with oil levels You are effectively increasing the volume of oil within the system of your car by adding a cooler. During refill, fill the car with the correct ammount of oil specified for your car. Run the car and check level. Then run the car up to temp and recheck, top up as necessary. You need to warm the car to make sure the thermo (oil)stat opens.

If you opt to fit the cooler infront of the rad. Make sure that the cooler sits flush against the rad with no gap If there is a gap make sure you use some form of ducting to seal between the cooler and rad.

Wing mounting is best because it cuts out any turbulant air and has no compensating for any reduction that positioning the cooler infront of the rad will cause to the cooling system.