don't know what else to try. The truck in question is the '97 Rodeo with 155k miles; runs flawlessly, and no oil burning/using issues, doesn't leak, etc. Thing is is this truck has seen a life of Havoline oils, about 2yrs ago a sound that 4 different mechanics have concluded to be sounds of friction has developed. It's not a loud sound, but still one I don't care for. It's not the sound of ticking lifters, fuel injectors, and the timing belt and tensioner are new. The rods have also been ruled out, via the fact that the noise has not increased, but yet in fact decreases as the engine oil gets miles on it. The uoa's that I've had done, show incredible numbers for an engine with 155k miles. Thing is, is the truck ONLY seems to really like 5w-30 Havoline blend - this is the only oil that it seems to like, but it shears out WAY too much after only 2k miles of 3-4k rpm's of mountain driving which I do a lot of in the summer. I've tried other 5w-30 oils, i.e. regular Havoline and Chevron, friction noise gets REALLY loud, as well as the 10w-30 Havoline blend. This is what doesn't make sense, the sound of friction is not related to viscosity, as it'll be quieter on one 5w-30 but loud on other 5w-30's. Last summer I tried a round of 10w-40 Havoline, in which it really quieted it as well, but I'd like to use an oil viscosity range that doesn't shear so badly. Still the odd thing is, after about 1-2k miles on the oil, the friction sound gets quieter; when I change the oil/filter at 3k it gets louder again. This is what I can't figure out. FWIW- I only change the oil every 3k because Isuzu engines like REALLY clean oil and have a tendency for their lifters to clog if not changed frequently enough. Also, oils I have tried are Rotella 5w-40, still leaves sound of friction, worse for about 3-4 days then gets quieter. GC - forget it, won't do it again, truck sounded REALLY bad. Regular 10w-30 Havoline (the oil the truck lived on for 148k miles) gives off a friction sound that I don't care to really hear, a sound I never heard before 2yrs ago. I've read that in order for one to bypass the sounds of friction is to use a thinner oil and one likely to have lessor amounts of zinc, but that combination could also cause more wear. My truck calls for 5w-30 only at below 0F temps and not in the heat of summer or with extensive highway driving. I'm about to dump in 15w-40 Delo for summer to see what happens, but is the 15w too much for a SOHC engine?? ANY thoughts are welcome, I'm at a loss of what's causing this sound.