Funny you should ask because I have the same situation with my 1988 vintage Yanmar 2GM20F and the Kanzaki tranny (sorry, but I don't recall the model number off the top of my head.) I have changed the ATF several times (I've owned the boat, a Pearson 33-2, for 4 years) and this past spring noticed what was on the plate on the back of the tranny. I have consulted various Yanmar experts, including Mack Boring at the boat show and a fellow named Chris Oliver in Annapolis, well known as a Yanmar guru. Neither one was all that alarmed over it because apparently the lube capabilities of the ATF are well within the range the Kanzaki requires. But both said to drain it and replace with SAE 30 wt oil as the manufacturer intended. But as with you, I am reluctant to change something that has been working all these years. Yes, I did consult the prior owner, who owned the boat for most of her 14 years before I bought her, as well as the mechanic who serviced the engine many of those years for him. The mechanic said it had always had ATF in it so he simply replaced it as needed. He never thought anything about it.
Despite history, however, I probably will drain and replace with oil this fall -- several times in fact to make sure I get all the ATF out.
BTW, you're the second Caliber owner I have encountered with this same situation.
SailorMitchSailing winged keels since 1989.
1.20.09 Bush's last day the end of an error !! Hopefully we still have a constitution and economy left by then.
"Compassion and tolerance are not a sign of weakness, but a sign of strength." The Dalai Lama
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I have but one lamp by which my feet are guided, and that is the lamp of experience. I know no way of judging the future but by the past.-- Patrick Henry.