Differences between EP90 GL4 and GL5

[Peter0987654321]

One of my vehicles that I run is a 1970 Series Land Rover. I've been told that I shouldn't put EP90 GL5 specification oil in the gearboxes because it will ruin the bearings etc., and should stick to GL4.

Is there are truth in this or is it just another urban myth?

Pete

 

[drwebb]

GL5 is a different spec, and is not 'backwards compatible' with GL4.

GL5 gear oil is a lubricant with very high extreme pressure performance needed for Hypoid gears where sliding and rolling contacts operate under very high loadings. This is achieved by incorporating high concentration of sulfurized additives. The downside to such high sulfur in oil is it tends to corrode yellow metals, so can ruin parts like brass synchronizers and some bushings. So any equipment needing GL5 performance is designed taking this limitation of the lubricant into account.

GL4 oils are much more friendly to yellow metals, and so are used in equipment where such high load sliding-rolling contacts aren't present. The designers of such equipment aren't as limited in the use of brass parts. So the bottom line is that if your equipment calls for GL4 it probably doesn't need GL5 performance, and you may be asking for trouble down the line if you use a lube it wasn't designed for.

 

[25362]

Very little can be added to what drwebb said.

The main functions of a gear lubricant are wear prevention, friction reduction between the mating surfaces, and to carry away the heat developed during friction.

Some comments on EP additives. EP means extreme pressure additives. These actually act by "corroding" ie, reacting chemically with the protected surfaces.

In more severe sliding and loading conditions, as in gears, simple adsorbed or chemi-sorbed anti-wear additives will eventually be inadequate to prevent severe adhesion.
If the temperature increase caused by surface friction is too great the adsorbed molecules will desorb and the protective film will no longer be present to prevent adhesion of the surfaces.

Thus, more powerful films are required. These films are provided by reaction of various chemicals with the metal surfaces, and most of these chemicals contain sulfur, phosphorus or chlorine.

The combination of local high temperatures, increased surface energy, and any catalysis or activation process that may take place, makes the action of EP additives more specific, taking place preferentially at the wear points where it is most needed.

Because the reacted surface has a lower shear strength, the sliding friction is markedly reduced, and the tendency to severe adhesion will also be reduced or eliminated.

Many thousands of compounds of P, S, and Cl have been checked and found successful, from the mildest phophorus-containing compounds through the middle active sulfurised types to the most reactive chlorine compounds.

EP additives are, in fact, selected to obtain the best balance between corrosiveness and protection for the wide range of systems in which they are used.

 

[Peter0987654321]

Thank you both very much for the detailed answer.

I guess that I'll be draining and re-filling the gearboxes with GL4 now!