The Copper Strip Tarnish Test (ASTM D130-10) is a standard test of a lubricant to indicate how corrosive it is to copper alloys that may be in a transmission. My restored 1989 Suzuki Sidekick has brass synchromesh transmission components that may be vulnerable to additives found in many GL5 lubricants. As a result, my owner's manual specifies a GL4 75W-90 or a GL4 80W-90 lubricant for the manual transmission and transfer case. Lubricants that are "yellow metal friendly" are hard to find. I perused through data sheets and called lubricant manufacturers to find copper strip tarnish test results for their products. Those lubricants that I found test results for are in the table below.
A brief description of the test can be found here: description D-130. The classification scale ranges from 1A (least corrosive) to 4C (most corrosive).
M/T FLUID & GEAR OIL SAE API CLASSIFICATION Amsoil Long Life FGR 75W-90 GL5 1B Dark Orange Amsoil Severe Gr SVG 75W-90 GL5 1B Dark Orange Amsoil M/T Fluid MTF 5W-30 1A Light Orange Amsoil MT & Gear MTG 75W-90 GL4 1B Dark Orange Citgo Citgear Std XD 75W-90 GL4 1B Dark Orange Delo Gear Lubric ESI 80W-90 GL5 2A Claret Red Delo Trans Fluid ESI 50W 2A Claret Red Red Line MT-90 75W-90 GL4 1A Light Orange Red Line NS Gear Oil 75W-90 GL5 1B Dark Orange Red Line 75W90 GL-5 75W-90 GL5 1B Dark Orange