Engine oil. For Lycoming the oil pump sucks the oil out of the sump and under pressure sends it to the oil cooler. From there it returns to the engine to be sent to various locations and ultimately drain back into the sump to repeat the process. There is more to the system than that but this enough information for this log.
There are two ports on the accessory gear case for oil to be returned from the cooler; one below the governor pump and one near the oil pump. Most folks use the lower one, but this engine was configured to use the upper, not sure why but for now I am leaving it that way. You can see the new lines running to the oil cooler location.
