Generally, synthetic lubricants have lower friction properties than conventional, non-synthetic oils do. Synthetic lube may still be preferable if the duty is severe or high temperatures are a concern. Synthetics typically offer superior performance in those areas due to a more durable molecular structure. The overall friction properties of the lubricant are also influenced by additives and modifiers blended in. Clutch type differentials typically need these modifiers. They help to even out the transition from static to dynamic coefficients of friction between the clutch plates. This, in turn, smooths out their operation and reduces chatter. However, it also means that they may slightly lower the overall differential locking performance as well.
Lubricants sold as “limited slip oils” have these additives premixed in. Be aware of this when making a selection. For OEM applications, the validation testing that I referred to above includes extensive vehicle handling and chassis dynamic tests. Consequently, the OEM oil (and differential performance) is matched to the intended handling behavior for that car. So, where does that leave us? Quite simply, here: use a good quality product, one that meets the needs of your ring & pinion gearing. Beyond that, think about what your specific needs are and choose accordingly.
