It’s such a small quantity and you don’t change very often, so just get what they recommend. It’s been too many years since I worked with Ford AWDs, but there may be a couple reasons for the lighter weight. If the system is on-demand or significantly rear-biased, the front axle will not produce as much heat. If it is demand-based, likely the only time it will truly be working all the time is in the snow, and the temps are cold enough the lower weight won’t impede fluid life. The rear axle may be doing the brunt of the work all the time, including high temp summers. That’s my take. It’s much easier when your AWD is 50/50 all the time, rain sleet or snow. Both my AWD transaxle and rear axle take 75w90. Easy peasy!