I’m a long-time Redline customer. I love the shifting feel and cold-morning smoothness of their MTL in my cars and have tried their Shockproof products without incident, but I tend to change fluids frequently with vintage machines.This is not in any way an indictment of the company or its products, they make great stuff, but a curious example of how modern lubes and older engineering sometimes don’t work well together.

A longtime Guzzi rider recently discovered that the Shockproof Heavy in the rear drive of his 1973 Eldorado had separated into a thin carrier oil on top while the buffer additive –meant to reduce impact wear on the surfaces of drive components– had settled into the sump and remained there.

The later Ambassador, all Eldorado, and some 850 Tonti-frame models (non-cush drive) have a removable finned sump plate for changing fluid and inspecting the rear drive. The removable sump plate is unwieldy and messy for changes, but it allows for a visual inspection of the gears, which I do every change. i can shine a light up in there and turn the wheel to inspect the teeth for excessive wear or chipping.

There is a barrier above the sump with small holes, which apparently prevents the Shockproof from being engaged by the gears and continually “mixed”, allowing it to settle into components over time.

One outward symptom is that the carrier oil, when unmixed, is thin (pourable) enough to seep out of the fill plug and the thin gaskets of the pinion mount. If you are using Shockproof Heavy and notice this slight seepage, I strongly recommend an inspection immediately, or before further use. It will also occur from overfilling; this is a fluid change that requires a measuring cup after a full drain, do not use the level plug at all.

In this case, the pinion, running only on the carrier lube and in a smaller quantity of lube, was found to be scored and effectively ruined. When I hold one of these small pinion gears in my hand and think about the tremendous forces transmitted via such a small surface area, I vow to keep good lube in there at all times. Shockproof Heavy would seem to be a very good solution for this model –since it does not have a cush drive– to help absorb driveline shock. But the design of the inner sump, made for period formulations, clearly thwarts that.

I have used Shockproof Heavy in an Eldorado rear drive and transmission, it did impart a smoothness. However, I stopped using it on my Eldorado several years ago, switching to conventional dino gear lube because of the more frequent changes I prefer. Nearly any GL-5 compliant oil will work acceptably if changed per correct intervals.

I suspect that this issue results from the design of this particular rear drive. The first loopframe rear drives, known as the Star drives for their compact star shape, may have been found to overheat in certain real-world conditions. Whatever may have been found back then, the next generation of Guzzi rear drives have the look of a strong engineering reaction. The case appears to have much more surface area, holds more lube, between 20% to 100% more depending upon which maintenance resource is consulted, and was designed with an extended sump with the aforementioned finned cover. My guess is that the intervening plate in the sump was meant to lessen foaming of early gear oil formulations, yet allow for enough thermocycling for cooling.

Subsequent Guzzi rear drives returned to the rounded shape –larger in size than the Star drives– with a drain plug and no extended sump or removable sump plate. I suspect, but have no data either way, that these would engage the Shockproof Heavy and keep it mixed if the bike is used routinely.