Most likely, however, if the tree got covered in oil after it lost its leaves and became dormant, it may survive if the oil can be washed off. Try spraying the tree with a very dilute soap solution (add about 10 drops of detergent to a gallon of water and mix well). Let the solution stay on the tree for about 5-10 minutes and then rinse thoroughly with fresh water. Repeat daily for about a week (you may see visual signs that the oil is actually being removed). Do this on days where the air temperatures are above freezing, so the solution won't freeze on the tree. You could also use a natural organic product called neme oil that is sometimes used for aphid control. You can try spraying it on the tree and then rinsing it off. Again repeat the process to ensure you get all of the hydraulic oil off. Do not use a pressure washer to clean the tree. Aspen bark is very soft, Using a pressure washer may strip off the bark and kill the tree. The washing process, together with natural snowfall or rain may help to remove the hydraulic oil and keep the tree alive.