Synthetic is said to last longer, so how many miles should I drive before changing tranny oil in my '77?

 

every year no matter what

or say every 10,000 -- or 15k if you want

or test at each change and adjust accordingly

 

The smartest thing you can do is have your oil analyzed at 6K miles (or one year as mentioned earlier) to see what is going on inside the case. I change mine every 5-6k regardless as I consider it cheap insurance. My 2005 997S specs said to wait until 20K before the first change and I dumped the "break-in" oil at 2K and changed it again at 8K. It's such a cheap and simple process I can't see any reason for going long periods. Dino oil is dumped at 3K so 6K for Synthetic still isnt bad.

My track car gets changed (Mobil 1) every 500 miles or twice per season but those are all trck miles..

Just my opinion..

 

Well, first of all, you shouldn't be running synthetic oil in your 915 tranny. I think the first two posters missed this. You should use Swepco 201, and change it once a year. 915 synchros do not work well with synthetic transmission fluids. (Synthetic is great for the 1987 and later G50 transaxles, though.)

 

WOW - I _did_ miss that. Good catch, Jack.

Swepco is good for your 915 - change it every few years or 15,000 miles - whichever comes first. Do it a bit sooner if you want to be ultra careful. You can get it analyzed just like with engine oil.

Be sure to loosen the side (filler) plug before undoing the drain plug. Get the syntheitc out of there fairly soon.

For more info search here and on Pelican.

 

WOW! Talk about reading the subject line and resonding too soon!

Thanks Jack, always on the ball and 100% correct!

 

After reading the recommendations in here and Pelican, I put Swepco 201 in my SC this morning. Immediately noticed a difference in shifting smoothness. Thanks again for the advice folks. I'll change it yearly.

 

I guess Gary & I are both pre-programmed to see synthetic and think engine oil - despite the pretty clear question.... again apologies, and glad it worked out for you.

There are some instructions on replacing plastic bearings in the shifter and coupler - search here & on Pelican -- should make a further difference in shift quality. You can also put new, better shifters on, or get any of several nice aftermarket ones.