A member of the system who has a combined total of 10 years or more of creditable service in the system and prior service credit at the time of retirement, and who was required to complete one or more periods of six months or less in the service of an employer participating in the system before becoming a member of the system, shall receive retirement credit for those periods of six months or less if the member, within 90 days before the effective date of retirement of the member, applies in writing to the board for that retirement credit and pays to the board in a lump sum an amount determined by the board to be equal to:
ORS 238.125