Gents, my gearbox incl. O/D unit was refurbished about 8 years ago. Til know I drove about 6.000 miles. While searching for a problem of the O/D activation I notices that there is "mud" in the oil filter of the O/D unit. Hence I changed the oil. My question is: how often should I change the gearbox oil? I thought that there is no need doing a regular oil change when drivin only a few miles per year.