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Introduction

Adding various additives into the engine oil and into transmission oils is a common temptation among enthusiast vehicle owners, both as a preventive measure against future wear and as a performance / fuel economy improvement trick.

It is also sometimes considered when an issue with the engine or transmission mechanics develops due to excessive wear of the internal components.

The purpose of this article is to provide general guidelines on when such additives should (not) be used.

The accent is on the adjective general, because there are various additive products on the market, with varying types and degrees of chemical composition, usefulness, compatibility, designed effects, side effects and operational risks!

Engine oil additives

The main purpose of most engine oil additives is:

• To reduce the friction between moving parts of the engine;
  • Additional side effect is a slight improvement in fuel economy as well;
• To "permanently" coat the moving parts of the engine, so that they have less friction while the engine is working "dry" from a cold start;
• To seal any imperfections in internal engine oil seals;

Some engine oil additives (example: Liqui Moly Cera Tec) have been highly praised for their positive effects (reduced engine wear, improved fuel economy, smoother engine sounds) in many circles in the motoring industry and enthusiast motoring forums.

However, an often overlooked side effect of most engine oil additives which reduce friction is reduced engine braking performance (a logical consequence).

That side effect might not be of any significance for (inter)city driving, especially in flat terrain regions.

However, engine braking performance can be quite important for an all-terrain vehicle driving application, especially on a hilly or on a mountainous terrain (descending steep hills).

Therefore, you need to weight the benefits of an engine oil additive against the loss of engine braking performance for your typical driving situations. Remember that Jimnys do not have a relatively powerful engine compared to their mass, so an additional loss of engine braking performance might be a significant disadvantage for some descent driving situations.

Transmission oil additives

The main purpose of most transmission oil additives is:

• To reduce the friction between moving parts;
  • Additional side effect is a very slight improvement in fuel economy as well;
    • This is only noticeable if the vehicle is primarily driven for very short trips (if the transmission is working cold most of the time);
• To "permanently" coat the moving parts in the transmission, in order to improve meshing between various gears;

Most transmission oil additives are based on MoS2 chemical compound.

It is very important to bear in mind that not every transmission oil additive is suitable for every type of transmission or every component of transmission (gear box, transfer box, differential). Super extra care and investigation is needed in order to avoid very expensive misapplications!

Additives in a manual gear box

It is generally advised to avoid putting any aftermarket-made transmission oil additives in a synchronized manual gear box (a design which is used in all Jimnys), unless a vehicle manufacturer specifically recommends a specific additive for a specific situation.

The main reason is that the gear synchronizers inside a synchronized gear box are usually made of "yellow metals" (a layman term) and that the chemicals in most transmission oil additives (primarily those which are MoS2 based) slowly eat such metals away.

Even if an additive does no such harm, its friction altering behavior can have unpredictable consequences on the synchronizers (and therefore on gear box's gear shifting behavior), because their principle of operation is usually based on having a specific pre-designed amount of friction between them and other moving parts in the gear box.

Additives in an automatic gear box

It is generally advised to avoid putting any aftermarket-made transmission oil additives in any automatic gear box, unless a vehicle manufacturer specifically recommends a specific additive for a specific situation.

The main reason is that the basic operating principle of an automatic gear box is based on the friction of the automatic transmission fluid. Since most transmission oil additives alter the friction of the oil or coat the moving parts themselves to alter their friction, its friction altering behavior can have various unpredictable consequences on the operation of an automatic gear box.

Additives in a transfer box

Since transfer boxes in Jimnys have no synchros nor any "yellow metals" (a layman's term), most manual gear box oil additives should be safe to use in a transfer box.

However, according to forum reports, the occurrence of worn gears in a Jimny transfer box has been very rare anyway. The part of the transfer box which is far more likely to fail first is the transfer chain, and it usually fails not due to friction wear, but due to physical stress (overstretching), which is usually caused by heavy-footed driving.

Additives in an axle differential

Compatibility of additives with differentials varies greatly depending on the design of the differential - if it is a plain "open" design, some kind of a "limited slip" design (LSD), some kind of fully lockable design or something else.

All Jimnys have plain "open" design of axle differentials.

Generally, an MoS2 based transmission oil additive should do no harm nor have any negative technical side effects when applied to the transmission oil in Jimny's axle differentials. Therefore, the decision to apply it or not is generally upon the vehicle owner.

Conclusions

There are three main general conclusions on the topic of engine and oil transmission additives:

1. If the goal of application is a preventive anti-wear measure, a better strategy might be just to regularly replace the engine/transmission oil more often than prescribed by the manufacturer, and only use high quality and completely suitable oils.
   • Read the wiki articles "Fluids and filters - proper selection" as well as "Regular maintenance intervals and consumables" for more info on proper selection of fluids and on prescribed and recommended replacement intervals.
2. If the goal of application is performance (vehicle power) improvement and/or improvement of fuel economy, consider in the following two factors:
   1. The risks of misapplication;
   2. The practical importance of reduction of engine/transmission braking performance when driving downhill;
3. If the goal of application is resolving an operational issue due to developed mechanical wear in the engine or transmission, then it is advised to first thoroughly investigate what the best (or at least the least likely damaging) additive product for your situation might be, and only then try it.

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Page last edited on 21/02/2019 by user Bosanek