Originally Posted By: fdcg27
For most OTR diesels, cold starts are a small enough part of the equation that the difference between a costly Grp III 5W-40 and a less costly Grp II 15W-40 is nil.
Given the huge sump capacity of these engines, and the very satisfactory life from an engine run on a 15W-40, there is probably no overall cost savings to be realized by using a 5W-40, and more likely the 15W-40 is simply more cost effective overall.
If significant savings in fuel consumption could be realized with the use of a 5W-40, along with significant improvements in engine life, you can bet that fleet operators would be all over 5W-40s.
The savings in fuel and wear aren't there, though, so most fleets use 15W-40 oils.
I think Doug Hillary would disagree with you. He ran a large fleet of OTR trucks in Australia and used Delvac 1 synthetic 5w-40 with change intervals between 50,000 and 60,000 miles because it was cheaper than 15w-40 changed every 15,000 to 25,000 miles.