I have a 2008 Mitsubishi Lancer Evolution 10, Factory spec is SAE 90 GL5 for transfer & Rear Diff.

The recomendation on the product selector is a non synthetic 80w-90 for both items. 

I have bought the GO80900025- but does not have the same consistancy as what I took out, and smells entirely different, my concern is that the oil recomended is twice as thick, and will damage my vehicle, particularly if it has a synthetic oil in there at present.

As far as I understand vehicle was previosly maintained by a Mitsubishi Dealer with Genuine drivetrain fluids used, or at least on the invoices supplied with vehicle. 

Is there a more suitable synthetic oil available from yourselves? Or is a premium mineral sufficient for the vehicle, and I am just thinking synthetic base oils are a more performance oriented item?