I owned a 2005 Infinit G35 and it has a manual transmission. At some point something strange was happening in regards to shifting.
Example: When in first gear, if I revved the engine to 3000RPM and shifting into 2nd, it would slightly grind. If I revved the engine highter to say 4500RPM, and then tried to shift into 2nd but not forcing it, at some point as the RPM's came down it would go into 2nd no problem.
So I then took my car for an oil change at the dealership, and the mechanic took the car out for a drive and when I got the car back everything was fine. They didn't even mention what they did, but whatever they did it wasn't anything elaborate since they didn't charge me and it didn't take any longer that a routine oil change (2 hours maybe since it is a dealership).
Anyhow, what possibly could the issue have been?