I use Mack Bulldog Premium Plus 10w30. It is around $15 a gallon, I get it from my work, and it is diesel rated. The best place to search for a deal on oil would be truck shops and parts stores, they usually have something on promotion.
This is taken from a document I found while searching for some info on this oil.
Quote:
Mack® Bulldog®
• Having the optimum viscosity at each operating condition will minimize overall engine friction and reduce fuel consumption. Two things are important: viscosity itself as well as viscosity retention
•Viscosity –A multigrade lubricant with a lower viscosity grade could provide improved savings
– For example, using a Bulldog EO-O Premium Plus 10W-30 compared to Bulldog EO-O Premium Plus 15W-40 would provide a better chance of capturing the potential fuel savings
•Viscosity Retention
– Viscosity can increase over time because of lubricant break-down and contamination. Soot particles, contaminants, and by-products of oil break-down can lead to oil thickening, leading to reduced fuel economy and compromise wear protection
• Mack 10W-30 products provide outstanding protection against lubricant break-down and contamination that promotes the potential for improved fuel savings from the outstanding viscosity retention side as well as from its lower viscosity grade
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