The 10W-40 issue goes back to issues GM had in the mid 80's.

About 10 yrs. ago I wrote Castrol, Valvoline, Quaker State and Pennzoil a letter on this matter. The factory service manual for the '95 Rodeo says that using 10W-40 can cause premature engine wear. Pennzoil and Castrol responded. They both said their 10W-40 would not harm my '95 Rodeo. I wrote 2 letters to Isuzu asking them to explain their stance and heard nothing from them.

I don't doubt that 20 yrs. ago it may have been an issue. I suspect the problem has since been solved.

I don't think that 10W-40 in itself is the issue, but what is the proper weight oil for your yr/make/model.

On another forum, a tech named Jake swore up and down that 5W-30 was the ONLY oil to use in the 93-97 6VD1 V-6 3.2 liter motors. Ambient temps. were not an issue. You simply use 5W-30 year round whether you live in Phoenix or North Dakota.

I've noticed that with 5W-30 in my '95 Rodeo, things are a lot quieter underneath with the motor idling.

An interesting topic no doubt.