On the "service chart" I have that the gas stations used in 'those' days , it states
Pontiac Firebird 1967-69
"service at time or mileage-whichever comes first

ev. 4 mo. or 6,000 miles
( oil filter , replace at first oil change and every second oil change there after. On 400 RamAir engine, filter loosened and tightened from under the car, lifted out and installed from under the hood.)

Crankcase, 1968-69 SE,SD API service motor oil , 1967 SE,SD,SC API service motor oil
5 qts except 1967 8 cyl.,6 qts and 1969 6 cyl 4.5 qts
add extra 3/4 - 1 qt when replacing filter...

above +20F, 20W-20, 10W-30, 10W-40,20W-4-, 20W-50*
0 F- +60F, 10W,5W-30,10W-30, 10W-40*
below +20F,5W-20,5W-30*
* 30 may be used abv +40F,5W-20 is not recommended for sustained high speeds.All RamAir
and H.O. engines, 30 or 10W-40 all temperatures.

maybe this will tell the tale...
of course its 'aftermkt' station service ,not dlrship...