Is Lower Viscosity Oil Really Better? 

Best oil and fair price.

I was thinking about this last dark cycle(night) that perhaps I shouldn't run T6 5w-40 in my '81 Turbo 6.2 Diesel as it probably has more clearance in the bearings throughout the engine and is more than likely more harmful than helpful due to the lower viscosity. I'm switching back to T5 15-40. Though i do have easier cold starts, improved performance and increased fuel economy by using the 5w-40it doesn't outweigh the added protection a slightly thicker 15w-40 can offer. Protection over performance.

I don't agree with the statement that headwind, driving style, or a tank of bad fuel will erase the benefits of the lighter-weight oil. If real-world conditions change, then both the heavy-weight and light-weight oils would see a decrease in fuel economy. Granted, the real-world variables that were mentioned certainly occur and impact fuel economy, but that is independent of the oil viscosity.

They have never lowered the weight of vehicles in 50+ years. Every year they cram more BS into every car and raise the weight. A 60's Mercury Comet weighs less than a modern Honda Civic. They promised huge weight savings with all the plastic in cars but just added more with other mandated crap.

You guys wont-way too much for that oil anymore