To avoid any confusion, there are 2 kinds of "friction modifiers". Ones reduces friction (for rear axles) and the other adds friction (for manual transmissions).

One type is sold in small bottles by GM and redline, designed to be added to plain differential oil to make it more slippery, and sutible for hypoid designs where lots of sliding occurs in the mesh. I add this to my diff to reduce lock-up .... my preference.

The other "friction modifier" is an additive included in gear oil designed for transmissions to make it grab the syncros better ... is kind of adds friction to the oil. I think redline was among the first to sell this trans oil, now most oems have their own .... GM, Chrysler, Honda (MTF), etc.

I found neo 75W90HD a bit better than redline MT90. Also think you need to change it with some regularity to keep the shifting crisp.

 

This is true.

However, Mazda specifies only "API GL-4 or GL-5, 75w-90 or 80w-90" as their recommendation for the transmission oil. All of the synthetic gear lubes I know of that are mentioned here are GL-5s, or even GL-5 and GL-4. So while it's quite reasonable to experiment with friction modifiers (friction adders, in this case), Mazda gives no indication that this should be necessary.

Dave

 

Good point, I should have mentioned that as it's more common to hear about the diff additives.

I guess my point is that non-synthetics most likely didn't need the amount of modifiers that synthetic requires. While I haven't seen the API specs, I don't think there are a required amount of modifiers. I believe the API specs are just for lubrication properties. I'm betting the older synthetics didn't have enough modifiers for proper use, kind of the same as the older synthetics not being good for rotary engines.

 

I recently had some in-depth conversations about GL4 vs. GL5 with an experienced engineer at Royal Purple.

Basically, GL-5 specifies a minimum level of EPs in the add pack. It doesn't limit how much, or what kinds, just a minimum. So it's quite common for a GL-5 to also meet GL-4 specs. I have not read the API spec myself either, but it was explained to me from a couple of sources that this is the case. So most synthetics are formulated to be both GL-4 and GL-5. Notable exceptions are Mobil 1 being a pure GL-5, and Redline MTL and MT-90 being true GL-4s.

Unfortunately, more EP additives are not necessarily better - and not all EP additives have the same side effects. The cheap add packs are loaded with reactive phosphorus and sulfur which do well with protecting steel gear surfaces, but attack the yellow metals. Expensive EPs used in most synthetics are not aggressive to yellow metal, but do still present the issue of excessive lubricity.

The funny thing is that Mazda isn't clear on whether this tranny works best on GL-4 or GL-5. PFS's comments suggest the GL-5 is not good, and that GL-4s may be better. Nissan has many gearboxes in service that supposedly can't tolerate GL-5, but supposedly it's hard to find GL-4 now and so the dealers pressured Nissan into allowing GL-5. With the difficulty of finding GL-4, maybe some of these manufacturers are recommending tranny slip modifiers to offset the use of GL-5.

Dave