Personally, I'd stick with a 10-40 full synthetic - especially with those temps. I've used 10-30 full synthetic Mobil 1 in a 3.0 for the last 400,000+ miles without issues, though.

I did have what sounds like the same thing with 'disappearing oil' in my 3.0L. No smoke, no leaks, nothing in the coolant. Nothing anywhere. Passed emissions tests with flying colors. I switched back to the Mitsu PCV valve (after reading some posts on this board) and switched to 10-40 full synthetic (high millage) Mobil 1 and mysteriously my oil isn't disappearing like before. I also had on and off ticking. Cold, hot, in the middle - didn't matter. Switching to the 10-40 helped - it's not as bad as it was with the 10-30. I'd bet that it'll get worse if you go with a thinner oil.

How 'HOT' is hot? If it's 100F+ degrees regularly, I'd be thinking of maybe running something slightly heavier than 10-40.

I wouldn't run a 'blend' unless I was desperate. What's your oil change interval? What's the price difference between a full synthetic quart and a blend quart? Here the cost is about 40-50% more for a full synthetic than blends. Its actually cheaper to run a full synthetic in the long run because a reasonable change interval on full synthetic in our engines is 8,000 miles. The change interval for blends is 5,000 and I definitely wouldn't push that.

I've got just over 450,000 on my 3.0L and I only change the oil and filter about every 8-9,000 miles. Been doing that since the day I bought it new off the showroom floor. Engine still runs like a top despite the weird missing oil thing (that seems to have gone away) and the annoying occasional ticking. <img src="/forums/images/graemlins/smile.gif" alt="" />

Edward