I've done some math and let's look at the numbers.
During winter operation this plant run one compressor RWB -134 with displacement of 790 CFM. Suction pressure is 5 psig and ammonia density at this pressure is 0.073 lb/ft3. 790 x 0.073 = 57.67 lb/min.
57.67 x 453.6 g X 60 min X 24 hrs= 37,667,000 g/day Assume that oil carry over is 1 ppm. It will be 37.667 g/day or 37.667 x 30.5 = 1150 g/month. Oil density around 0.9. It means at 1 ppm of oil carry over, 1278 ml of oil will be drained every month. To drain 3.8 liter of oil every month, oil carry over will be 3 ppm. I think it is not bad for 20 years old plant. I'm not sure how people get 2 tablespoons of oil carryover every year, because it is 1500 times less than 3 ppm.