The Auburn Gear limited-slip differential and Select-A-Loc Electronic Locking Differential designs have been extensively tested with high-quality Non-Synthetic 80W 90 hypoid oils treated with limited-slip friction additive. Three (3) oz. of Auburn Gear additive 504102 (sold separately) will treat one (1) quart of oil. To avoid differential clutch chatter (noise) and for optimum performance, use the oil and additive described above. Use of other additives and oil types may cause differential clutch chatter. We do not recommend synthetic oil.