A 5L sounds very strange for that size transformer. If actually want you to take that much, just be sure to convey to the client that you will also be billing them additional costs to replace the sampled volume of oil. You are correct in being concerned about lowering the oil level too low. And not only do you need positive pressure on the tank when you complete the sample, the transformer needs adequate oil volume to account for oil volume shrinking during cold weather. I routinely see low oil alarms from substation transformers during cold snaps due to oil sampling during hot weather.
I have see the need to take way more than 5L that from substation transformers. In order to get representative sample, any oil volume in piping must be drained before taking a sample. In one case, each phase of my LTC had separate piping about 20 ft long. Basically every time we do an oil sample at that substation, we also have refill the transformer conservator. I am now including a requirement in substation transformer specifications for the conservator to be sized large enough to account for 20 years of oil sampling at the manufacturer's recommended sampling schedule.
