Just had a rebuilt trany installed in my 95 BMW 525i by my BMW specialist. It is the French made GM trany of dubious quality when new. The rebuilder (wholesale to shops only) filled it Castrol dex/merc and said it should only be filled with the same product when serviced. Do not change brands. Also do not use synthetic. Any opinions on this? They also suggested a drain and fill after 3,000 miles. Not necessary though. Opinion?