I want to change my tranny fluid over to synthetic, but don't want to go through the hassle of doing a full drain. Can you mix synth and dino? I was planning on dropping the oil pan, putting a drain plug in, new filter/gasket, re-torquing, then changing the fluid every few thousand miles until it starts to look clear. It hasn't been serviced recently that I'm aware of, and I'd like to go to synth to increase it's life span.

I have an '82 XJS, with a TH400. It has about 140k miles on it.

Hi mathohns,
I have an '86 (project/experiment) & '87 with the same tranny.
If you haven't serviced it or don't know when it was last serviced, best to do it right the first time and change everything, the filter, gasket & fluid will always be good.
Good opportunity to make sure it's in working order; check the colour of the fluid and if there are any debris in the filter. If it was driven hard in the past with your mileage, the clutch plates/bands may start to wear. Both of mine started slipping before that age.
Cheers
Peter

With the prices for synthetic transmission fluid so high, what are you saving by avoiding the cost of the flush? Doesn't the flush go for about $75 + tranny oil? At $8/quart, you would certainly go through more than an extra 8 quarts of transmission fluid while doing the mix and drain method.

I would just bite the bullet and pay for the flush at a responsible and honest mechanic who wont charge you for the flush, but provide the mix and drain.

If its worth switching to synthetic, its worth doing right.

I power flush mine transmissions by removing the correct trans cooling line at the radiator, gets a heck of a lot of it out. You need to be careful how its done so you don't cook anything.
I have not done the 400 this way (lumped mine),so can't speak to that.

Never bothered with synthetic on the trans, especially on one that has some age on the seals,I'd just put dino in the trans.