Oil pressure changes

Just a quick question:

I recently changed to a synthetic blend oil based on the ambient temperatures here in AZ (sometimes gets up to 120 degrees in the day). Using Quaker State 20W50 because it never gets colder than 85 degrees here and I was afraid of a lower viscosity oil getting thinned out in the heat.

In the winter I was using Castrol 5W30.

In the winter, my typical idle (900 rpm) oil pressure on startup was 50 PSI and would drop to 20-30 once the engine warmed. With the new oil, typical idle pressure is 70 PSI and drops to 40 once warm. Does this sound normal? It looks really high to me...

My understanding of the 20-50W bit is, it's a 20 weight oil that protects like a 50 weight.

Same deal with the 5-30W.

Before I swapped over to synthetic in my (new at the time) 2002 F-150 w/5 liter I asked about 5-30W vs. 10-30W - both synthetics - on a board that's run by a well known tech writer.
He said - in so many words - that the 10-30W will give better protection.

In my opinion, manufacturers recommendations are not always for the consumers best interest, but for the corporations best interest.

Keep in mind the 5W oil debacle on the Buicks a while back.
Quite a few of them wiped out bearings and not too long after we find that the 5W recommendation was to better the CAFE bit for the company. (CAFE = Corporate Average Fuel Economy.)

Following the manual is the way to go in most cases, but you have to do some thinking for yourself as well.

Quote:

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Originally posted by DennyW
5w30 to 20w50, normal reaction. Remember, that 20w50 when you put it in, or it's a cold sit overnight. The weight is closer to 50 weight. The engine heat activates it, and it drops to the 20 w, and as engine heat goes up the oil thickness to 50 starts.

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denny, thats sounds backwards to me. i thought the oil got thicker hot and thinner cold. on a cold morning the oil should be thinner so the motor can turn over and the oil will flow freely, as it get hot it would thicken to help keep the oil pressure up.

venom, thats a big change, i guess its telling you its doing what its suppose to. go with the 20w40.

lt1s10, yes it was a big change, but my ambient temp was radically changed also. I moved here from North Carolina and looked at the chart below regarding recommended ambient temps and the oil you should use:

http://www.chris-longhurst.com/carbi...s/saerates.gif

The temperatures here in Yuma are so extremely hot that I figured I needed an oil with a much higher viscosity to prevent the hot oil from thinning out and not protecting my motor. Here is a quote from the website I used to pull the information:

"The proper viscosity is the single most important criteria of a lubricating oil. The basic performance of machinery is based on the viscosity of the lubricant. Viscosity is, if you like, the resistance to the flowability of the oil. The thicker an oil, the higher its viscosity. The chart on the right (graphic above) shows a rough guide to ambient temperatures vs oil viscosity performance in both multigrade (top half) and single grade (lower half) oils.
Multigrade oils work by having a polymer added to a light base oil which prevents the oil from thinning too much as it warms up. At low temperatures, the polymers are coiled up and allow the oil to flow as it's low number (W number) indicates. As the oil heats up, the polymers unwind into long chains which prevent the oil from thinning as much as it normally would. The result is that at 100°C, the oil has thinned only as much as it's higher rating. Think of it like this: a 10W30 oil is a 10-weight oil that will not thin more than a 30-weight oil when it gets hot.
The viscosity index of a lubricant is an empirical formula that allows the change in viscosity in the presence of heat to be calculated. This tells the user how much the oil will thin when it is subjected to heat. The higher the viscosity index, the less an oil will thin at a specified temperature. Multi-viscosity motor oils will have a viscosity index well over 100, while single viscosity motor oils and most industrial oils will have a VI of about 100 or less."

Once we are finished with the hot season here, I will be switching back to the lighter oil. Thanks for the input!