Originally Posted By: Dak27
I also run a 12.7 Detroit Series 60 motor in my semi. I've been using 15W40 Phillips 66 Guardol semi-synthetic for the past few months. Because I pull quite a bit of heavy loads out of Idaho, which put my gross vehicle weight close to 80,000 lbs, I feel better about running a heavier oil for the majority of the year, especially when the outside temperatures are very warm. My loads frequently have me going into Texas and across the South.
I do use 10W30 in the coldest months of the Winter. I'll be using Guardol 10W30, starting in late November. I'll switch back over to 15W40 either in late March or early April. I notice the 10W30 does help for quicker cold weather startups.
I also pull some heavy loads. Last two weeks, nothing in the box under 43K. Same stuff for my customers I normally pull. The first year I tried the 10w30, I did as you mention... used 15w40 in summer. Then the following year, I did 10w30 all year. No indication either via oil pressure or used oil sample that it was a problem using 10w30 in the heat of summer pulling heavy loads. Detroit's Lube Manual does not lay out any variables in ambient temp regarding using 10w30 vs 15w40. 10w30 is allowed right on up thru the ranges for the 40 weights irregardless of load in the motor. It is just that so many of us have been conditioned to use 40 weights that it is hard to wrap our minds around the idea that a 10w30 would do the job just fine. I have figured it out, so it doesn't bother me anymore.