Average temp doesn't show how toasty some of the localized hot-spots are in a transmission. But, the words "synthetic" and "reasonably priced" usually don't play well together.
Whats the sump size and recommended interval?
http://www.mobil.com/USA-English/Lubes/PDS/GLXXENCVLMOMobilfluid_424.asp
# Heavy-duty transmissions, differentials, final drives, hydraulic systems, power steering systems, wet brakes, PTOs and hydrostatic drives
# Recommended in off-highway industries including: agriculture, construction, and quarrying
# Recommended for top-up and refill of systems
# Suitable for commercial transmission applications requiring Type A (Suffix A), Dexron and Type F fluids. Do not use in passenger car automatic transmissions
# May be used in applications requiring API GL-4 or SAE 80W grade lubricants in all gear applications except hypoid gear designs
# Outstanding replacement for engine oils recommended for hydraulic and transmission systems