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When servicing or checking the oil level in an overdrive transmission it has two service plugs that need attention. Both service plugs are on the passenger side of the transmission. The main gear box plug is easy to access — it has a 5/8″ square head threaded plug on the side of the main gear box. The overdrive unit aft of the main gear box also has a 5/8″ square head plug. The overdrive service plug is a little more difficult to access behind the transmission to frame mount above the frame cross member.
When servicing a freshly rebuilt transmission or changing the oil the overdrive unit should be serviced first, Fill it with oil until it overflows the filler hole. Oil does flow very slowly between the overdrive unit and the main gear box. The main gear box is then filled until it overflows. I recommend letting them set level for 24 hours then remove both filler plugs to insure they are not overfull. Oil should be at the top bottom edge of both filler holes. This procedure will insure that both units have an adequate oil supply. It is important that these units be service properly. If the overdrive unit oil level is low the output shaft drive gear may not receive proper lubrication which could result in failure of the gear which drives the speedometer as well as the overdrive governor.
The recommended oil for an overdrive transmission is 90W Mineral GL1 gear oil. Modern gear oil with higher GL ratings should not be use because some of the additives are corrosive and could be detrimental to the brass components. The synchronizer blocking rings in standard three speed and three speed overdrive transmissions are brass.
Most auto parts stores do not carry GL1 Mineral Gear oil, but it can be ordered by most major auto parts stores. One of those is NAPA as shown in the attached picture.