There are several things that can cause slow engagement to reverse, what concerns me more is what transmission fluid was used, it should be ATF type F. Also the fluid level should be correct, drive the car so the transmission gets to normal operating temperature, park on the level, shift through each gear and back to park, make sure it engages each gear before going to the next, once you are in park, leave it running, pull the dipsticks wipe, re-insert, pull and read both sides, the lowest one is where the fluid actually is, if it is low add some, remember from the add make to full is only 1 pint (1/2 quart). It is better to have it a little under the full than to overfill it, overfilling can cause air to be mixed with the fluid, and this causes a multitude of problems. If your level is too high to begin with, then I would, remove the inspection cover on the bell housing, rotate the flexplate until the drain plug for the torque convertor is at the bottom or close, put a drain pan under it and remove the plug, just drain a little out, like if it is maybe a pint overfull, then drain about a pint out, and then put the plug back in. Then recheck the level using the procedure above. Good Luck.