How to change tranfer case oil?? - 1998 GMC Sierra

I cannot help you with quantities, but I expect it to be no less than 1 gallon.
Therefore purchase at least 4 QTS. of Heavy Duty EP 80/90 Gear Oil.
I suggest this due to age of vehicle, also as you consider a Transfer gear oil change, you change the Differential as well.
For this also, an EP 80/90 is adequate. The joice of Mineral Oil or Synthetic is yours.
Regards, DT

The transfer case uses ATF, the same type as used by any Toyota Automatic transmission

Sounds like a defective electronic shift motor for the transfer case. The diff and t-case fluids should be replaced every 30K but will never turn on serv 4wd light.

the oil leaked, either through an oil seal or where the case is joined, also indicating a leak on where either a gasket or gasket sealer should be applied.

refill the oil, drive it and check for where the oil is leaking from.

Replace the oil seal or redo the transmission assembly and ensure that proper gasket sealant is used where needed.

SEA 75W90 GL4 SYN both trans and tranfer case

yup! i just did my 02 jag. the tranfer case is going down baby...just to change the oil gasket...hehehehe..

change the senser on the tranfer case and maybe the automac. locking on tranfer case

Check the fluid level in there. If it was low for an extended period, change fluid. Whining is often caused by low fluid level, but is also a symptom of internal wear. If fluid change does not help, it needs to be rebuilt or replaced.

the transfer case uses the same ATF as the trans.

You may have pressure build up in the tranfer case and its going out the weakest spot which is the seals.Who changed the seals?Your transfer case may have a blocked breather if fitted