Gear Oil for the Model A

I am going to say I am no expert, the following is how I understand the gear oil works for the A.

First we must understand there are different gear actions.  There are the straight cut and helical (I think that is the correct term for curved gears) cut gears.  The straight cut gears are mostly a direct pressure movement with little swiping.  The helical gears have a swiping or shearing motion between the faces.

Modern oils are really designed to work with the shearing motion found in the curved gears. 

The steam cylinder oil does not get squished out under heavy pressure.  The modern gear oils are designed for a shearing action and are more likely to get pushed out allowing metal to metal contact.

When engineering the Model A the Ford designers built the parts to take advantage of the 600w type oil.  This oil likes to coat and stick to stuff.  It does not flow as well so you do not have to tightly seal some areas.  So let think about the A.

The transmission is designed to leak the thick oil some.  How do you think the u-joint is lubricated?  The grease does not do much but create a tight area to keep the heavy oil around the U joint.  The front bearing does not matter since it is fully covered.  The straight cut gears in the tranny need the heavy pressure oil and the none syncro straight cut gears shift better with a nice heavy oil. The rear axle gears are also straight cut and have a large amount of pressure on them.  They really need the proper oil.  Have you ever noticed the insides of the axle housing how they are generally not rusty?  The heavy oil really gets around and just sticks to the housing.  This prevents the rust that is more likely to happen with a car that is not driven much and is likely to get condensation.

The steering gear is another area that needs the heavy oil. Here is a place were some modern lubricants may be a better choice.  There is a lubricant made by Penrite.  I do not have the exact product number on hand.  This is a very thick semi fluid grease that sticks to the gears and is not likely to just leak out.

The following is a post giving the modern equivalents of the 600w oil:

Posted by Marco Tahtaras from ( on Saturday, August 09, 2008 at 9:38AM :

In Reply to: Mobile 636 gear oil posted by Bob from ? ( on Friday, August 08, 2008 at 10:35PM :

Mobilgear 636  => Discontinued =>This is the replacement Mobilgear 600 XP 680

Energol GR-XP 680

Castrol Alpha LS680

Omala 680

Meropa 680

Here is some more info on gear oil.

Re: 600W Thickness -- One (1) Answer

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Posted by H. L. Chauvin on Nov. 26, 2011 at 16:34:03

In Reply to: Re: 600W Thickness, Transmission Gear Grinding
posted by louis on Nov. 25, 2011 at 17:19:35

Hi Louis,
Your former question made sense when someone new to this Forum reads "all" of the many "different" former 600W discussions in the attached archives.

It appears most "salespersons", (parts suppliers & oil compaies), offer the 1930's 600W substitute, but as Marco noted in the archives in the late 1990's, they are "not" all the same.

Many articles appear on experiences of switching to thicker transmission oil; thus eliminating driver's not experiencing grinding of transmission gears.

Gear oil thickness, (resistance to flow), is measured by several methods.

Kinematic Viscosity per ASTM D445 is one standard method indicating the milimeters squared divided by time in seconds at 40 degrees C, or 104 degrees F, which is closest to trasmission gear oil viscosity after attaining operating temperature.

For example, per ASTM 445, water has low viscosity, honey has high viscosity.

The recommended BP Energol 680, Shell Omala 680, Mobile Extra Helca Super Cylider Oil 680, have ASTM 445 measured viscosities, (40 C), of a high 680; Texaco Mepora 680 indicates a thinner 646; however, Valvoline 85/140W drops down to 395, & Castrol 85/140W is thiner at 369.

One of the "most" scientific test conducted on Ford's 1930 recommended 600W appears to be a gentleman who wrote that he had an old sealed can of
Ford's recommended 600W.

After trying many gear oil substitutes, his highly scientific lab test indicated that the 600W was extremely thick, almost like melted tar, & equal to the above mentioned 680 viscosities.

His test, he dipped his finger in each & watched it flow!

It appears if one uses recommended gear oil with a viscosity of 680, & gears continue to grind, begin looking somewhere else.

Hope this helps 1930's future 600W research.