Final answer:
The synthetic E-500 gear lube is the OEM-approved lubricant that would produce the longest service life in a standard transmission.
Step-by-step explanation:
The OEM-approved lubricant that would produce the longest service life in a standard transmission is the synthetic E-500 gear lube. This is because synthetic gear lubes, like the E-500, are designed to provide excellent lubrication and protection under a wide range of operating conditions. They have superior heat resistance, oxidation stability, and film strength, which helps to reduce wear and extend the service life of the transmission.