Fine print in the new federal gas tax law could force Virginia to let motorcycles use the car-pool lanes of Shirley Highway and I-66 during rush hour, transit officials believe.

Section 167 of the mammoth new law, which President Reagan signed yesterday, stipulates that no federal funds can be used to build or resurface car-pool lanes unless those lanes are open to motorcycles, too.

"It does seem to indicate that the state doesn't have a whole lot of choice if they accept federal money," said David Erion, executive director of the Northern Virginia Transportation Commission, after a commission meeting last night.

Motorcyclists have sought access to Shirley Highway's car-pool lanes for years, arguing that their vehicles are small and fuel-efficient, meeting the goals the lanes were created to achieve.

The state has turned down these requests, largely out of fear of creating a precedent to allow vehicles other than the four-person car-pool onto the express lanes.

With I-66 already built and not likely to need resurfacing soon, it is unclear how the law would affect it. Similar questions remain for Shirley Highway.

The new law does give states the right to exclude motorcycles if it is shown they would create a safety hazard on the restricted lanes. Last night, the transportation commission called on the state not to change the car-pool restrictions without a thorough study of the safety issue.

In other action by the commission last night, Arlington County Board Vice Chairman John G. Milliken was appointed as one of Northern Virginia's two voting members on the Metro board. Milliken, a Democrat, will replace Arlington Board member Dorothy Grotos on the Metro board.

At Metro earlier in the day, Prince George's County Council member Richard J. Castaldi was sworn in as the county's new voting member at Metro. He took the place of David Hartlove, a County Council member who was defeated for reelection last year. Castaldi is to take over the Metro board's rotating chairmanship later this month.