GL4 is an older spec than GL5 and when our mini toys were manufactured it's possible that GL5 didn't even exist. Most gear oils are rated for both GL4 and GL5 specs. I think the manual transmission MAY shift a little better with GL4 because it was designed to work with the GL4 spec, but either one should be sufficient. Other than Redline, I'd suspect it's difficult to find a GL4 oil that isn't also spec'd for GL5.
