I've just come across the same problem, 424 out of stock everywhere and dealers are telling me 428 has replaced it. Looking to do my WJ transfer case.
If you check the product spec sheet though, the viscocity is different so im worried this aint the same oil and reluctant to be the guinea pig especially when these boxes are known to be picky !
https://www.mobil.com/en-nl/commerci...mobilfluid-428
https://www.mobil.com/en-uy/commerci...mobilfluid-424
Now im no expert, but these numbers aint the same folks!
Mobil 428: Density @ 15 C, g/ml, ASTM D4052 = 0.866
Mobil 424: Density @ 15 C, kg/l, ASTM D4052 = 0.880
Mobil 428: Kinematic Viscosity @ 40 C, mm2/s, ASTM D445 - 59
Mobil 424: Kinematic Viscosity @ 40 C, mm2/s, ASTM D445 - 55
Mobil 428: Viscosity Index, ASTM D2270 = 141
Mobil 424: Viscosity Index, ASTM D2270 = 145
Mobil 428: Flash Point, Cleveland Open Cup, °C, ASTM D92 = 243
Mobil 424: Flash Point, Cleveland Open Cup, °C, ASTM D92 = 198
Mobil 428: Pour Point, °C, ASTM D97 = -48
Mobil 424: Pour Point, °C, ASTM D97 = -42
Maybe we have an oil / hydraulic fluid expert in here though...