Hello, my name is XXXXX XXXXX I can help you with your question.
To better answer your question I need just a little bit more information about your problem.
Does your mower go full speed when you first start mowing? How long does it take before it starts to slow down? Is the slow down gradual over a period of time or all of a sudden? Are there any unusual noises when it slows down?
I want to give you the most complete and accurate answer I can and this information makes that possible.
Let me know when you have time.
Based on your description it sounds like you have one of two different problems.
The transaxle in your L120 was not ment to be serviceable. There are very limited repair parts for it and Deere suggests that it be replaced when drive components fail. Consequently they did not make the transaxle easy to do maintenance on either.
In order to check the oil level you need to remove the vent plug (#28 in the attached diagram). The oil should be only about an inch down from the top of the vent plug hole. You will find that the vent plug itself is very difficult to access and even more difficult to add oil if needed. The recommended oil is Low Viscosity HyGard oil. If you slightly over fill the oil level it might leak out the vent plug after the transaxle gets hot but won't hurt anything.
If you find that the oil level is ok then it is most likely that the transaxle is worn internally. Rather then discard the transaxle for a pricy new unit you can often get a little more life out of it by draining the oil and filling with 10w30 engine oil. The 10w30 engine oil is thicker and won't leak past the worn internal components as fast. The catch is that the transaxle is not equipped with a drain plug so the transaxle must be removed from the tractor and the oil drained from the vent plug #28 hole. This is not a permanent fix but it might get you by for another mowing season.
If this has helped to answer your question please hit the green "Accept" button. If you need further assistance simply respond to this message and I will do my best to help you out.