So lately (the last 5,000 miles?) I'd noticed a gradual, slight change in shift quality on my '13 1LT M6 - like the engagement of the gears was less of a "jumping over a gate" feel and more of a gradual push into gear. I'd changed to Amsoil 5w30 synchromesh (2 qts.) at 30,000 miles, (currently at 38,200) and wondered if the thin fluid wasn't just flat too slippery for crisp synchro engagement, but I pulled the console apart today and adjusted the shift cables and judging from my 5-mile test drive just now it's back to the way it should be. I'm taking the car on a 1,300 mile trip next week, and provided my adjustments do indeed make a lasting difference and no problems arise I can make a pic-by-pic of this process (I haven't noticed one on the forum yet) because any assessment of transmission condition/fluid behavior will be skewed if the shifter has worn-in without being readjusted.