The Seiko caliber 6309 is a vintage automatic workhorse movement that was found in many Scuba Diver models, seiko 5, and dress watches in the 1980’s.
In Seiko’s own words:
In old Seiko documentation, they refer to the 6309 as a Highly Reliable Movement:
The highly stabilized accuracy and high reliability established for the 61XX series has been incorporated into the movement.
Also mentioning easy servicing:
Disassembling and reassembling procedures and serviceability have been improved largely by:
Employment of a new balance hairspring holding device (aka no stud for the hairspring since it clamps directly in the terminal)
Decrease in number of parts from the simplification of the movement structure.
Decrease in number of new parts resulting from interchangeability of some parts with Cal. 61XX series.
There are different variations of this movement, including Seiko caliber 6308A (17J), 6319A (21J), 6347A (moonphase, 23J), 6349A (23J).
In an interesting thread on the Military Watch Resource forum comparing the Seiko caliber 6309 to the ETA caliber 2824, shortwave said:
I have a 25 year old 6309 7290. It hasn’t been cleaned in at least 10 years. It advances 1 to 2 seconds in 12 days. I’ve never seen such accuracy in any manual watch. The ETA in my Marathon, gains seconds a day.