X2 on what Doclouie said.
It's even ok to mix different types of engine oils like synthetic and dino oils, even when they are not the same viscosities. For the ATF used in the t-case, you could even throw in ATF+3 if you needed to without problem. Just don't use other types of ATF besides ATF+3 or ATF+4. Other ATFs like Mercon are totally not ok to use.