EP in GL4 vs GL5

[JSeery]

There seems to be a lot of misunderstanding in what is going on between different GL ratings on gear lubrication. The common gear lubes available today are GL-4 and GL-5 and one of the major differences between them is the amount of EP additive. The higher the amount of EP in the lubricant the better it is a handling high friction between mating surfaces.

"The designation API GL-4 denotes lubricants intended for axles with spiral bevel gears operating under moderate to severe conditions of speed and load or axles with hypoid gears operating under moderate speeds and loads. These oils may be used in selected manual transmission and transaxle applications where MT-1 lubricants are unsuitable."

"The designation API GL-5 denotes lubricants intended for gears, particularly hypoid gears, in axles operating under various combinations of high-speed/shock load and low-speed/high-torque conditions."

There is no need to use GL-5 in the banjo year flatheads, it is just not required. However it is not going to hurt anything in the rear axle. The problem occurs when "yellow" metals are involved, such as in the transmission. This is because of how the EP in the gear lube works.

EP (Extreme Pressure) additives can be different formulations, a common one is sulfur/phosphorous. The sulfur/phosphorous additive works by forming a sacrificial coating on the materials it comes into contact with. This coating is dependent on both pressure and temperature at the mating surfaces.

Now a side note here, the temperature element here is at the mating surfaces. One of the major functions of any gear lubricant is to carry this heat away from these surfaces. The temperature at the mating surfaces will be much higher than the total temperature of a rear axle or a transmission. Consider an engine, the temperature inside the combustion chamber is much higher than the surface temperature of the engine itself. In the same way, metal mating surfaces under load will be much higher than the surface temperatures.

The temperature and pressure of the mating surfaces determine the amount of sacrificial coating on the components. During use under load this sacrificial coating peals off the components in contact and protects the underlying metal. The higher the EP in a lubricant the greater the protection and the higher the loads it can handle.

Now for the issue with "yellow" metals. A component, such as synchronizer, also get coated with the sacrificial material and it also peels off under load. However in this case the sacrificial bond is tight enough that it pulls some of the softer metal off with it. This is at a microscopic level and is a very small amount, but over time even this small amount begins to add up. Think how many times the synchronizer goes through this cycle of being coated and then having the coating pealed off. With time the synchronizer will not mate properly with it cone.

GL-4 also contains EP additives but at a lower level than GL-5. At the levels used in GL-4 the sulfur/phosphorous sacrificial coating does not form as strong a bond on the "yellow" metal surfaces and is able to perform it sacrificial function without removing the base material. Because of this it does not provide as much protect as a higher level of EP would, but that level of protection is not required! The loads experienced in transmission does not require high EP levels and protection of the soft metals is more important.

GL-4 will preform its intended function in a flathead transmission while protecting the soft "yellow" metals. It will also function well in the rear axle.

GL-5 is not required in any transmission using synchronizers and with damage them over time. It can be used in a banjo rear end because of the lack of "yellow" metal, but is not required because of the type of gears used.

Anyone wanting to make corrections or add additional information please contribute.

[Talkwrench]

Nope, think you covered it well.. GL4 is known as 'mild EP' or that's how Penrite label it. Its about the only stuff we can get here in Australia suitable for early trans.

[49fordv8f4]

GL5 probably wouldn't be safe in a Columbia axle though, as they have bronze washers in them.
Mark

[redmodelt]

For example, 1948 F2's also have bronze thrust washers in the Timpkin rear end.

[JSeery]

Good point!

[donald1950]

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good post....

Grainger has GL-4 Valvoline oit. part number is 46KK52

[50fordcoupeman]

Not hard to get here in Oregon. Most parts houses will order GL-4.

[Dak Rat]

The GL-4 lube I bought at the local NAPA store states on the back of the jug "will not harm yellow metal parts found in older manual transmissions" Good enough for me--I'm using it.

[J Witt]

Nice post, good info, to the point. I used Lucas oil stabilizer straight in the transmission of my Model A (no synchros) and it worked very well. Quieter and better shifting.

However I still have mineral oil in my '36 transmission, of unknown composition. You've now got me thinking about changing that out in case what is in there is GL-5.

So, GL-4 is a safe choice that is known to function well, but how about the Lucas oil? The package doesn't list any GL ratings at all, so I assume that it is just a mineral oil with no additives. Sounds like the EP additives aren't really necessary in the transmission or are they beneficial for bearing and gear tooth wear?

Comments?

[JSeery]

The EP additives are beneficial, but not required. There are also the GL-1 & GL-3 options.

[RalphG]

I thought I had it figured out last year but on reading the label of my local supplier's MP gear oil I see it meets specs for GL4 and GL5, which might or might not be safe on "yellow metal". Here is a direct quote from their spec sheet which sounds like I should be safe using their MP Gear oil.
DESCRIPTION
SONIC MP Gear Oil is formulated with CGT (clean gear
technology) to reduce deposits, increase fluid and gear
life. It is an ashless oil designed to eliminate chemical
reactions with yellow metals such as brass. SONIC MP
GEAR OIL provides superior lubrication in passenger
cars, farm equipment, heavy and light-duty trucks, off
highway equipment and industrial gear applications.
APPLICATION
SONIC MP Gear Oil is recommended for use in any
application requiring a heavy duty, extreme pressure
gear lubricant. It is approved for API Service GL-5 and
meets the performance of Mil-PRF-2105E. Special
additive chemistry also makes it suitable for top up and
complete refill of limited slip differentials.

[flatford8]

Originally Posted by J Witt

Nice post, good info, to the point. I used Lucas oil stabilizer straight in the transmission of my Model A (no synchros) and it worked very well. Quieter and better shifting.

However I still have mineral oil in my '36 transmission, of unknown composition. You've now got me thinking about changing that out in case what is in there is GL-5.

So, GL-4 is a safe choice that is known to function well, but how about the Lucas oil? The package doesn't list any GL ratings at all, so I assume that it is just a mineral oil with no additives. Sounds like the EP additives aren't really necessary in the transmission or are they beneficial for bearing and gear tooth wear?

Comments?

Isn't Lucas Oil Stabilizer like STP? I would think that would be kind of thick, but I live in a much colder area of the country than you do...... Mark

[rotorwrench]

If it meets the mil spec MIL-PRF-2105E then it is the same as API GL-5 rating. I'm sure it exceeds GL-4 rating so that would be kind of a misleading statement if that is on the package or container.

It seems there are more than enough confusing and misleading statements out there to go around these days. To truly be API rated for GL-4 then that should be all they can put on the container. The product can't be both and be legitimate in my book.

The Lucas safety data sheet is either a sham or it truly has no additives in it other than plain straight mineral oil of a higher viscosity. This definitely resembles STP oil treatment except they state is has zinc now. I assume they are referring to ZDDP. I don't think it originally had any back in the day.

[JSeery]

A key here would be the statement that it is recommended for extreme pressure heavy duty applications. Now why would you need that in a flathead era transmission? Some gear lube suppliers claim they are buffered or contain a different EP additive and so on. It just seems odd to me that you would want to use something designed for a heavy duty extreme pressure application in the mild environment of a flathead transmission.

[jake197000]

ive used both for years never seen a problem with yellow metals ever,

[J Witt]

Ordered a gallon of CRC Sta-Lube GL-4 on Amazon today -- $24, free shipping, two days delivery if you are on Prime. As mentioned, what was in the transmission was uncertain anyway and fresh oil never hurts anything.

Thanks all for the info,

John

[Larkens]

Originally Posted by jake197000

ive used both for years never seen a problem with yellow metals ever,

That's good to know Jake!

[JSeery]

Always a gambler. There is almost no topic you can post on that doesn't get the "I've done it for years with no problem". Guess there are some really lucky folks out there.

[19Fordy]

Mr.JSeery "nailed it" clearly and concisely.
"You need not more than GL4".

[Crankster]

I've no doubt that the EP additives can damage yellow metals but it must take a while. The 3.03 manual gearbox in my slick specifies 50 wt mineral oil in the manual but not knowing any better - the clerk said "here, this", used standard 90 wt gear oil for both differential and transmission. That was 20 years ago and a lot of miles and regular changes.