I'm currently on my second oil change with Rotella T6 5W40 Synthetic and have a occasional "missed shift into second gear". It is definitely rider error in not "crisply shifting. If you are paying attention to operating the 6 and not daydreaming the trans works as designed. I've had several "heavy cruisers" and one required a fluid change on the hydraulic activated clutch to improve shifting.

Not sure but after nearly 20K miles of riding I may change the DOT 4 on the 6. Probably wouldn't hurt and may actually improve things