I have a 1990 Corvette with the ZF 6 speed manual trans and only 16,000 miles and want to know any reasons not to use synthetic fluids in the motor, trans, and rear dif. I have been told it will cause leaks, anyone have any experience with this?

Welcome to the forums...

Who said to not use synthetics?

I put some high $$$ auto tranny additive in mine when I did a flush, and it hasn't gagged after 3 months (only about 800 miles).

Sometimes, flushing chemicals, like pre-oil change flush, might solve dirt accumulations near a seal, and a leak develops. So ignorant people say "It caused a leak".:shakehead