You have to determine if the S10 has a locker or a Limited slip.
You can use 75W-90 in both the front and rear axle, it will not make a difference what so ever from a 80W-90.
GM Cars has Limited Slip Differentials and need an additive
GM Trucks have locking differentials and don't need the additive.
Don't confuse the 2 as they are completely different G80's. Use of the additive in the truck LRD will have a negative effect.
LOCKING DIFFERENTIAL LUBRICANT (SERVICE INFORMATION) #91-4-109
SUBJECT: LOCKING DIFFERENTIAL (G80) LUBRICANT - (SERVICE INFORMATION)
VEHICLES AFFECTED: ALL LIGHT TRUCKS EQUIPPED WITH G80 REAR AXLE ALL YEARS
Some light duty trucks equipped with locking rear axles (G80) may exhibit rear axle chatter, especially when turning a corner from a stop.
This condition of alternate engagement and disengagement of clutches in differential assembly is usually caused by contaminated axle lubricant.
To correct this condition, drain and refill the rear axle with SAE 80W-90 GL5 (P/N 10950849). The use of any additive in locking rear axles (G80) is not recommended.
Rear axle additives are designed for use in limited slip differentials which are normally installed in cars. All light duty trucks equipped with RPO G80 make use of a locking differential and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.
VEHICLES/COMPONENTS INVOLVED: