Manual Trans & Diff oil

07 Colorado.  The owners manual calls for 75W90 synthetic in the diff, 89021-806 (manual trans fluid) in the trans. Am pretty sure the trans fluid is not synthetic.  What about using synthetic oil in the trans.

Most transmission fluid is synthetic now. If it was natural refined oil, it would lose its viscosity over time and not protect as well. Theres also nothing in the Colorado transmission that can really be damaged by using synthetic fluid anyway. I say use it. It shouldn't make much difference either way.

Thanks for your thoughts.  My main concern is the synchronizers not being able to grab.