I've opened the mine by watching youtube videos. Is not hard as you say. It's easy but it needs about 3-5 hours work.
I've used Synco Lube grease for mine A5000. It was a very good choice. After two months of intensive use still quiet keys
By the way, you have to regrease the inner side of all keys too, and the contact surface between black and white keys like in my pictures.
That said, those who think that the guys at Yamaha have fixed the problem in the new models are sorely mistaken.
The black keys on my new PSR-A5000 were noisy right out of the box.