I was going to change my diff and tranny fluids pretty soon, but I'm not totally sure what to do with the old stuff. Can I just dispose of it with my regular engine oil? I want to get rid of it correctly. Let me know what to do. Thanks.

 

it is ok to mix it with regular engine oil, service shops do it all time.

btw its good that you care about the environment

 

Yes...you can just dispose of it with your used engine oil. They're all petroleum products.