Disclaimer: The following post contains a metric s#$t load of opinion. Take it with a grain of salt.
Hummmm, the only time I have heard of changing fluid causing problems is with an auto transmission or switching from conventional to synthetic engine oil.
Either way, change them!
If simply changing the fluid is going to cause a problem you are not that far away from a failure anyway. I would much rather have a reliable vehicle.
Now, lets qualify my first statement. the problems changing fluids can cause are related directly to "junk" built up on seals in both an auto transmission or an engine.
When changing the transmission fluid you are putting in new fluid with a fresh additive package. If the seals are only holding pressure internally due to a nice layer of junk on them the seals will start leaking internally after the new additive package cleans everything up. The transmission can then exhibit all kinds of issues like problems shifting or slipping. Once again, if a transmission is only working due to a layer of dirt, I want to know about it before it fails on the side of the road.
As far as the engine goes it is not so devastating as the trans. Synthetic fluid is exceedingly good and cleaning sludge and junk out of an engine (which is not a bad thing). In addition the molecules are of equal size and smaller than conventional oil. This being the case the seals on the engine will get all cleaned up and start leaking like crazy. But this is only if the seal is going to fail anyways. And unless you are not paying attention and don't keep the engine topped off with oil you are certainly not going to blow up your motor. (The above can happen with fresh conventional oil but to a lesser extent and only if the engine has been SEVERELY neglected.)
With other fluids I am not aware of issues changing them out and would highly recommend doing so.
Some people might call me a tree hugger here with the following statement and that is fine but definitely get all of your oil leaks fixed. We are having too many problems fighting the environmental nuts out there trying to close down our trails. Oil leaks and environmental contamination are frequently sighted as reasons to close our trails. If there are no leaks they have nothing to bitch about. And besides, it is much nicer to work on a gunk free rig than a grease ball.
So, in conclusion, change them all!
Cheers,
Rufus
P.S. just my opinion but if a mechanic told me not to change fluids...I would RUN!
1st, I am a big fan of synthetic, use it everywhere you can.
If you have a 73 and it is stock (verify EVERYTHING as these rigs have been changed ALOT by the time we get to them.) You most likely have the following.
chev 350 use 10-40 motor oil. I like mobile 1, amsoil, etc.
turbo350 auto trans. dextron III or higher ATF (IE dextron IV would be fine)
203 transfer case. This case requires straight 30 Wt motor oil. don't use gear oil or ATF. a multi weight like 10-30 should work fine as well.
rear and front diffs. use any 90Wt gear oil.
Brake fluid. Definitely bleed the brakes and flush out all the old fluid. standard DOT 3.
power steering. Nothing special here. standard power steering fluid.
