Hey guys, I am going to pop the top bolt of my differential and check it and change it today. I have a lot of "whine" sound coming from the rear. I have had it on jackstands and have gotten reasurace from several people that it is not wheel bearings. The sound persisit whether in drive or in neutral. It gets louder with speed. Anyway, me not having a lot of money has decided I should change the fluid first all. I know the fluid was replaced about 5 years ago. Anyway my question is, should I use synthetic. I know everyone has gone over this a lot. But I have a boot that is cracked pretty bad but not leaking. Basically I would hate for synthetic in some way to cause the boot to fail quicker b/c of it's cleaning properties. I basically just want to avoid any problems that I could prevent. I am okay with putting in regular gear oil but if the general consensus is I should use synthetic than that's what I want to do. I do not run any other synthetic fluids by the way.
Thank-you

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1985 Mercedes Benz 300SD, TMU
2015 Chrysler 200S