Two guesses- don't take the first one as an insult-

1- Make sure you're not overfilled on oil. I know it sounds goofy, but it can happen. Especially if you go to a quick change place- I did once upon a time and they put 5 quarts into a motor they 'were certain needed 5'. It didn't- it only held 4.

2- You may very well have valve stem seals that are leaking, or valves that need adjusted. The exhaust valves are towards the front of the motor, so if you are backing at a fairly quick speed you may be pulling oil into the cylinders. How do your sparkplugs look? Are they pretty blackened?

What type and weight of oil do you use? (synthetic, 5W30, etc.)

kt