5W-30 or 10W-30 for summer?

[Gandalf]

Now that it's winter, I've gotten to thinking about summer oil for my Vibe.I bought mine new in July '05, and at 1000 miles, changed the 'factory' OEM oil for 5W-30 Castrol GTX and a Fram premium oil filter.The manual recommends only 5W-30 oil. However, from having several previous cars with oil pressure gauges in them, I know that in hot weather, 10W-30 has significantly higher oil pressure than 5W-30 oil.I know on some other cars which recommend only 5W-30 oil, my dealer uses 10W-30 in the summer, although I can't get a straight answer from them on the Vibe.I got the usual bloody lie that changing my own oil voids the warranty from several people, and some others said use only 5W-30, while a few said if it was their car, they would use 10W-30 during the hot months. I've *never* let anyone change the oil in my cars; I've always done it myself, so the dealer won't know what oil I use in it.I keep thinking of inching along in rush hour traffic with the AC on in 95F heat, and thinking about that thin 5W-30 oil.Of course many parts of the USA regularly get 100F and higher temps, and I just can't help thinking that 5W-30 oil isn't going to provide the protection that a heavier oil will.What's the consensus on this issue? I know that the 5W-30 oils available today are much superior to 5W-30 oils of 20 - 25 yeas ago, but still.....? TIABTW: I only use Castrol GTX, and change oil every 3000 miles; sometimes less in the winter when the oil gets dirty more quickly.

[djb383]

It takes more horsepower to pump thicker oil through an engine, thus horsepower is wasted when trying to pump thicker oil. High oil pressure does not mean better lubrication. Thicker oil won't flow into the ultra tight tolerances of todays modern engines as well as a lighter oil will. The critical point of good lubrication and when the most engine wear occurs is when the engine is cold and most of the oil in down in the pan. A light oil will get to the top of the motor quicker and that is what manufacturers want for optimum wear protection. Hundreds of thousands of miles and hours are spent by manufacturers testing engines under extreme operating conditions and from this testing is how they determine which is the best weight of oil for their particular engine. You can't go wrong by following the Vibe owners manual and it says 5W-30 year round.

[the1stacekiller]

Been using Mobil 1 5W30 in three cars for a combination of 500,000 kms .. in the winter -37 degrees to the summer +40 Degrees... no problems, no leaks, no overheating, no rough starts.. NOTHING.. I have even contemplated switching to 0W30 (stock in some Porches) for better fuel efficiency, but the cost is still high..cheersMartin