Marinemaster servicing

[Mountain]

Hi all,

I’ve been wearing my Marinemaster 300 recently, but it seems to be gaining about a minute and a half a week.

It has the 8L35 movement and the monocoque case, so the movement is only accessible for regulating through the crystal.

My question relates to ease of service: Is this a job any competent watchmaker should be able to do, or does it need to be sent back to Seiko?

Thanks very much in advance!