Sometimes after I put the car into "P" it is hard to move back into the "D" position unless I pull up the handbrake after putting it into "P". It seems to occur on surfaces with slight inclines mostly.
It does not happen if I keep my foot on the brake and move the lever.
I have changed the transmission oil but it did not help.

 

When you park it without using the ebrake (which to me is a bad idea regardless, but especially so on inclines), the weight of the car rests on the parking pawl inside the trans. When you pull it out of park via the shift lever, you're also trying to pull the parking pawl away from the gear. With the weight of the car on it, it makes it more difficult to get into gear.

What I always do (as a force of habit) when I park is slide selector up to neutral, yank the ebrake, foot off the brakes to let the car's weight rest on the rear brakes, then put it in park.