Pressure oil

Alejandro, is the underside of your aircraft dirty with oil?  If so thats likely your oil venting problem. Jeff B is referring to the ice safety cut notch in the vent line- that should be done as well. Do not allow the oil tank vent line to protrude into the slipstream under any circumstances. Regarding your oil pressure concern, I just returned from flying and my 912UL, 200 hours tach time, built in late 2018, maintains oil pressure between 55 and 65 psi (4-4.5 bar) at engine oils temps from first start of the day to 212 Fahrenheit and rpm from idle speed to 5,500 rpm at an OAT of 24C.  Again, I am not a certified Rotax technician but I would not be concerned about your engine oil pressure.  

Thanks Jeff, I will implement it, it is not done. I will inform you of the results

Thanks Jim, indeed, I have traces of oil on the underside of my Tecnam P92. I will implement Jeff's information and tell you the results.
Thank you all

Alejandro,

Cutting this notch should protect against ice and also bleed off and any vacuum that might be created by the vent line being exposed to the slip stream, as long as the notch is inside the cowling. And yes, the Rotax illustration shows the vent line protruding a bit past the cowl. That being said, I agree with Jim that it is better to stop the vent line short before it exits the cowling to keep it out of the slip stream completely, and then also cut the notch.   

I see oily and exhaust residue on almost all planes ki annual. I use a water based biodegradable product called "Oil Eater" to wipe it off. It won't hurt any aircraft skins and when you spray it on the residue or grime will be running off before you wipe. It's  the best product for this that I've used in 30 years. I found it when I was flying helicopters. I also use it to clean up hangar dirty floors and use it on the stove hood at home to get cooking grease off. My wife loves it. You can buy it in a spray bottle, a one gal bottle or like me a five gal. bucket.

You're going to have residue on the bottom of the plane because of exhaust vapors and oil tank vent vapors.