More grist for the mill:
Here's a manufacturer claiming a 1% fuel economy improvement for heavy duty trucks by going with its synth gear oil vs. mineral (of a different weight - duh), or "other" synth oils:
Quote:
Roadranger FE Axle Lubricant is an API GL-5 extreme pressure gear lubricant designed for improved fuel economy in heavy- and medium-duty applications, as compared to typical petroleum 80W-90 or synthetic 75W-90 gear lubricants.
Benefits of Roadranger® FE 75W-90 Synthetic Axle Lubricant:
Better Operating Performance – 1% measured fuel savings (validated through fleet testing with an independent testing facilityutilizing SAE J1321 methodology)
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source:
Roadranger FE 75W-90