T.S,

I changed my tranny oil in my '78 bus because I didn't know the
history of the bus. By the looks of the drain plugs I'd say they
had never been changed. Yes there is a magnet on the drain plug,
but mine looked like Bart Simpsons hair do when I removed it -
quit a lot of metal bits were attached. When I put in the new oil,
which was most definitely correct and by the book oil, the tranny
was always stiff in first and second gears until the whole lot
warmed up. This tended to make me "crunch" the first to second
change - which isn't very good for the tranny!

In my opinion the most important reason for changing the tranny
oil is so that you at least know what it has in it and how much
it has in it. New, clean oil of the correct amount will ensure the
correct operation of the tranny (assuming it works ok first) and
hopefully add to the life of the tranny. After all of that though
I think it comes down to how the vehicle is driven. eg. My FIAT has
had little or no synchromesh on first and second since I bought it -
six years ago. I just take my time driving it and it never crunches.

Conclusion:

I would change the tranny oil and would change it at 20,000 mile
intervals, which for me would be every five or so years.

Just my experiences,

Lee.
L.S.Griffiths@Physics.Salford.ac.uk