Can I go back to using 10w30 oil in my 1990 GMC Suburban or stick with 10w40 oil?

[TrapperL]

Some of these posts are going on the MOM board at the lab in the morning. What babble! Jeez. In regards to what makes a synthetic. A synthetic can be an ester which is an alcohol and usually a fatty acid reaction. They are generally highly stable but don't handle moisture at all. These are in the general classification of Gp V. They do not do well in engines that see short heat cycles. You have the GTL base oils which are a gas to liquid technology. These are generally known as the PAOs , polyalphaolefins. These are classified as a Gp IV base oil. Generally they have pretty much been phased out after Katrina. Most of the synthetic oils that were PAOs are now either a Gp III or a mixed base oil. PAOs are now expensive and the ethylene gas used to make them is scarce. After Katrina, the PAO production was pretty much knocked out and formulators had to come up with new formulas which turned out to be cheaper. None of the oil makers or blenders ever went back to it. The Gp IIIs are most of what you find on the market today as a synthetic. This is a liquid and gas to liquid technology made possible by Chevrons ISO/SYN process. Mobil has a similar process called Raffinate. It's made from crude and just like any of the other base oils except for the esters comes out of the ground. Any hydrocarbon based oil comes out of the ground. Like the PAOs, the Gp IIIs do not mix with the additive packages. Some form of a binder must be used to hold it all together or wear is accelerated. The binders used are either an AN or ester in most motor oils. Today, you'll find a lot of the GP II base oils advertised as synthetic. All that's required is that the molecule does not happen in nature. Since even the Gp IIs are processed very similar to the Gp IIIs, the oil companies have been using it as a synthetic base. And just so you know, no synthetic oil will add one mile to the life of your engine. The base oil does not add any lubricity at all with the exception of the esters and so small you nor your engine will ever know it. At one time, back in the pre 90's the synthetics were known for heat tolerance but those days are gone as conventional base oils have made tremendous strides while synthetics are the same. If you look at any oil wearing any of the Ford specs on the bottle, it has been double time tested the requirements of API at 302F which your engine will never see. It cannot shift out of it respective viscosity range and high heat deposits are half that of API. The best oil out there wears the Ford specs in the car makers recommended viscosity. Properly maintained there are fleets on the road using Walmarts cheapest oils and many of the units are over 300,000 miles. Buy what ya like, you paid for it, maintain it how you want.

In regards to the winter weight. If you have cold start conditions you need to be looking at the CCS rating of the oil (Cold Crank Simulator). Not all 5w oils are built the same and some 10w will out perform the 5w in extreme cold. Don't assume that because it has a lower winter number that it's going to be the best. Always defer to the CCS of an oil if you have start up temps below 0F.

Beware of most any oil that have an extreme viscosity spread like that of a 10w-40. Using a common base oil like a Gp II or Gp III, the spread is not covered by the Viscosity Index. The formulators will load it with polymers to make it mostly because it works for them and it's cheap. Expect the engine to sludge if you run it too long or have short heat cycles as the polymers are noted for falling out of formulation. A 25 count spread is about it for making the viscosity spread without using many polymers.

[snofarmer]

Some of these posts are going on the MOM board at the lab in the morning. What babble! Jeez.
And then get ready to get snickered at.
Why? because you have gone over the top.
JMO


Whu? Because they all hold info that can be backed up by the oil industry.

[Mech-Mech]

Originally Posted by Grandpa Pipes

As long as you let the engine idle just a bit when cold started you should be OK. If I were you I'd short change the oil back to 10w-30 at about 3,000 miles. No real harm can be done as long as you don't mix a straight weight and multi-viscosity.

That said know this.........Your cars engine is engineered to run best , and longest , on either 5w-30 or 10-30 NOT 10w-40!!

Run to thick an oil and you will cause loss of power and possible oil starvation of the bearings at every cold start which will lead to MUCH shorter engine life.

Facts speak for themselves...
I have 1990 GMC 1500 Sierra with 5.7 L engine that I got at its birth. (July 1989)
I put the first 60,000 miles on it using mfg specified oil and viscosity of 5w30, at which time I changed to 10w40. (figuring that it must be broken in by now) Warranty ran out at 50,000 miles and it was now paid for.
I then changed to Pennzoil 20w50 at 114,000 miles and have not used anyother viscosity since.
I now have just gone over 287,000 miles and it purrs like a kitten.
I will also note that except for the first 60,000 miles, I have changed the oil and filter AFTER i had at least 6000+ miles, and many, many times much more. But NEVER EVER at 3,000 miles.
I regularly pull 6000+ lb horse trailers now in my retirement, and would not hesitate to drive it coast to coast tomorrow.
So, when I hear comments like changing viscosity is harmful, well, just maybe you are speaking of newer, more troublesome trucks and not the well built trucks of the 80-90's.
Just saying that "facts" are so much more dependable than the spoken word.

In its lifetime I have replaced (myself) these items.
trans fluid and filter replaced every 100,000 miles by a shop, I'm almost due again
New Front rotors and bearings at around 151,000 (had those done)
1 muffler (Midas)
4 or 5 starters
2 alternators

1 water pump, and 2 or 3 thermostats
all front steering related bushings
1 idler arm
1 ignition lock
most recently (last 5 years) I've done (in my retirement, and on the driveway)
high press switch on A/C pump, accumulater, orifice and air exchanger
retrofitted my A/C from 12 to R134 and use the same original compressor
replaced the fan under the dash
replaced front calipers and all 4 flexable brake lines
replaced the headliner (that was fun)
and replaced both "U' joints
and yes, there have been replacements of "other" rubber items such as belts, tires & hoses
(i've replaced the wires, plugs, cap and rotor exactly 4 times, once just 4 months ago)
Also one grill and fender because someone would not get out of my way. (I fixed that myself, except for the paint) ALL else on my GMC is original.
I lube the zirks every 4th or 5th year myself after seeing that almost NONE of the shops actually do what you pay them for.

There is such a thing as over worry and over maintenance.

I have broken down only three times in my trucks lifetime.
2 flat tires, and one alternator. Each time I replaced them on the side of the highway.

BTW, it's entire life up to 2008 it was used by me in my own construction company, sometimes carrying up to 35 sheets of 1/2 inch A/C plywood at a time (that is well over the 1/2 ton weight limit of my truck)

Believe what you want, or just believe the truth. I don't care. I sleep easy every night.

When is my GMC supposed to die from the harm and neglect of using 20w50 instead 5w30 ??

[need4speed2012]

Just drive it then change back, no big deal and no flushing.