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So I thought it would be a straightforward matter to drain and replace the oil in my '41 110 rear until I got under there and noticed there is only a filler hold and no drain. Thinking I could do it by cracking the rear cover off the pumpkin, I was surprised to see there doesn't seem to be any rear cover -- no visible bolts.

So, what's the right way to drain the oil? I suppose I could pump it out with a hand pump but that doesn't seem efficient and I was hoping to get in there with some brake cleaner and blast it all clean before refilling.

Also, was planning to use StaLube 80W90 Gl-4. Is that the right stuff? My manual just says straight 90W.

Thanks!

The drain plug is at the front side on the differential carrier casting.

Thank you, BDeb!

Then maybe I'm mistaking the drain for the filler plug. In which case, where do you fill it? I don't see any other plugs on the carrier casting.

Here's what I see looking from the front of the car to the back. I only see that one square bolt near the bottom. It looks too high up to be a drain as opening it would still leave a lot of fluid below that level.

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DO NOT USE GL-4. ONLY USE GL-5 OR EP GEAR OIL.

85W-140 is a good overall weight and can be used in the transmission, overdrive, and steering gear box.

Level and drain plugs are located on the front of the differential housing, on the driver side. The one you show in your photo is the drain plug. The level plug is located above it.

(o{}o)

Thanks, JW! I used Stalube SAE-140 GL-4 in the newly rebuilt transmission per the rebuilder's advice.

I figured GL-5 standard didn't exist when the car was made so GL-4 would probably not hurt anything in the diff but as long as there are no "yellow metals" in there, I will take your advice and use GL-5 for the diff.

The book does specify SAE90 for the diff year-round. wouldn't the 85-140 be too thick at operating temperature?

Many of us here, have used EP/GL-5 85W-140 gear oil year 'round for many years with good results and no problems due to the oil.

(o{}o)

+1 for GL-5 85W-140. At the expense of slightly reduced fuel economy (maybe a fifth of a mpg), you'll get longer wear out of your differential because metal to metal contact will be almost completely avoided. My rear end shop recommends that grade in everything he does.

To those who think it would be too thick, remember: 140 is one step higher in viscosity than 90. For reasons passing understanding, gear oil viscosity skips from 90 to 140. Click on the link below for a comparison chart. Gear oil and motor oil viscosity are measured differently, in fact a 90 weight gear oil is about the same Saybolt viscosity as SAE 50 motor oil.

Oil Viscosity Chart | Bob is the Oil Guy

JW,

I just filled the rear with Valvoline 85W-140 High Performance Gear Oil.

Can I use this in the manual trans as well. It says on the side of the bottle "for NONSYNCHRONIZED manual transmissions" and I know my transmission is synchronized.

I've had straight SAE-140 in there, but now that it's getting colder I'd like some multigrade to leave in year round rather than changing between 90W and 140W twice a year.

I find the higher GL numbers too slippery for use in transmissions; they tend to make the synchros less effective.
Get some 90 weight GL-1 from NAPA or Tractor Supply.and use it for the transmission and or overdrive only. Or as you can see from the handy chart, buy some straight 40 weight engine oil and pour it in. I use either of these in my cars.

Packard stick shifts are by no means delicate flowers. There is no need for any particular wonder oil. The high GL numbers are formulated for the sliding action of hypoid gears. There are none in any Packard transmission. In fact, extra thick oils, or oils thickened by long use will not adequately lubricate the needle bearings in the countershaft and are a death blow to R9 overdrives. If you wish to be very diligent, change it once every 10,000 miles or so.