A plea to the knowledgeable: Conventional vs. Synthetics

[Hib Halverson]

This thread seems to have skated around the three questions posed in "Bolisk's" long first post. But...that's the nature of "oil discussions" on Internet forum sites. The oil companies, no matter what kind of oil they make, just love that their product is the subject of so much dialog.

Let's review "Bolisk's" three questions:

1) Synthetic’s leak from the seals.
2) Synthetic’s cause limited slip/posi units to chatter (i.e. make noise).
3) Synthetic’s do not work with the tolerances of the older components.

First, synthetics can leak from seals but...modern seals are no more prone to leakage with synthetic than they are with petroleum-based lubricants. That is, if the seal is: incorrect for the application, installed incorrectly, defective, worn or the structure or device it's sealing is worn, the seal will leak regardless of the base stock of the oil. On the other hand if none of those issues are present, the seal will prevent leakage, regardless of the base stock of the oil.

Now...that's not always been true. Up until the mid-'80s or so, there were seal materials--mainly plastics and rubbers--which were not compatible with the base stocks used by many synthetic oils back then. The result was seal degradation and eventual leakage. That is from where the perception that all synthetic oils cause seal failure comes. Today, seal degradation in old engines with original gaskets and seals, is not as common because some lubricant makers have altered their formulations to be more "old-seal-friendly" but, the problem still does occur. IMO, if you want to switch to synthetic in a really old engine, you ought to change most of the gaskets and seals, anyway.

So...if you've got an engine, transmission or axle that dates to before the mid-80s and its gaskets and seals are original, there is a chance there will be leakage.

Second, synthetics can cause limited slip noise or chatter but...so can petroleum-based oils. That statement is ridiculous in its wide range. Some limited slip differentials use multiplate wet clutches and those are the type most prone to noise or chatter. The cone type limited slips (none are used in Corvettes) can also be problematic. Other types of diffs--Torsens (a limited slip which do not use clutches) lockers or ratcheting diffs (which are not limited slips, anyway) are not susceptible to lubricant-driven noise or chatter.

As to whether or not synthetic gear lubricants, inherently, cause chatter...the only ones that do are either not formulated properly or are incorrect for the application. That said, there are other reasons a lubricant can cause limited slip noise or chatter, but they are not specific to the base stock of the lubricant they are more dependent on the additive package in the lubricant, especially friction modifiers. For example, if you replace a petroleum-based gear lubricant which has 3% friction modifier with a synthetic lubricant having 1% FM, you may have chatter with the synthetic but it's not a fault of the synthetic base stock, it's a problem with not enough additive. You could replace a petro-based lube with another petro-based lube which has less friction modifier and have the same result: noise or chatter.

Third: the idea that synthetics won't work with tolerances present in older engines, transmissions or axles is B.S. Whether or not a lubricant will work with a given clearance is dependent on the lubricant's viscosity not it's base stock.

An example: take the modern T56 six-speed in a 2010 Vette versus the ancient M21 four-speed in a 1970 Vette. The modern transmission uses ATF, a very low viscosity lubricant. The Muncie uses an SAE 90 gear lubricant a high viscosity lubricant. It doesn't matter what the base stock is--petro. or synth.--if you put ATF in the Muncie it will fail and if you put SAE 90 in the T56 it will fail.

Same is true with engines. With a stock or near stock bearing clearances, if you replace a 5W30 or 10W30 oil with the SAE60 or 70 used in a Top Fuel motor, you're going to have problems. Newer engines still use a 5W30 or 10W30 and, depending on the duty cycle, come factory-filled with either petroleum-base oils or synthetic oils.

With respect to just engine oil, what engines really need synthetic?

Any engine factory filled with it, should have it.

Any engine that's run hard, ie: drag racing, road racing, very hard use on the street, will benefit from it. To be more specific, if the engine experiences sustained oil temperatures above 250 degrees F or any operation about 265°F, it needs a synthetic. Regardless of oil temperature, if the engine experiences loads far higher than it was designed for--such as a stock engine with a supercharger or nitrous oxide--will benefit from a synthetic oil.

Any engine which is run to extended oil drain intervals should use it.

Most other cases don't require it.

Can there be a benefit in using a synthetic in duty cycles other than those with high sustained oil temperatures? Yes but most using synthetic

 

[Bolisk]

Sounds like you've already got your mind made up, and have made some broad assumptions and generalizations to help convince yourself that you're right.

I'm in the complete opposite camp by taking the position of 'show me proof that they're better' and have been asking this for decades. No luck so far.

Other than a longer change interval, I know of no practical, real world advantage. On the other hand I know of no disadvantage other than cost, so use what you like.

I can see how you would think that, based on my comments. That was not my intent. I actually still waffel back and forth. I use mobile 1 in the early spring and late fall, then switch to Rotella 15w40 in the summer. . .when the oil is not so thick.

The reason my my post comes accross this way, is because we all know that conventional oil works. There is no argument there. I want to know why synthetic's don't.

Eaton says to use conventional. . .I just want to know why.

-JR

 

[Vettehead Mikey]

I have no idea why Eaton says that- but they don't make the engine. I'd follow their instructions for the diff and posi unit and not worry about it.

 

[Bolisk]

Just thought I would close this thread off.

Much to the dismay of some, I decided to roll the dice and try synthetic in my differential. I contacted Red Line, and asked them what was the correct gear lube to use in my differential. They told me that the 75w90 gear oil (NOTE THIS IS NOT THE 75w90NS gear oil, as the NS named gear oil is for the Muncie transmission) was the correct gear oil for my car, as it had the proper amount of additive (for the eaton posi) already added to the oil. By the way, I was VERY impressed with their responsiveness, and willingness to help identify the correct products for my car. They were very knowledgeable.

Before using the oil, I opened the diff, drained all the existing oil, and cleaned out the diff to check for wear, cracks, or any other signs of stress. Next I removed the side yokes and inspected them for ware. After finding that they were ok, I replaced the side yoke seals (as suggested by many on this forum) greased the side yoke bearings, and replaced the yokes with new C clips. I then installed a new end cover gasket with the proper sealant, let it sit for a couple of days (in order to ensure that it dried completely, then added the new gear oil.

I am happy to report that I have ZERO leaks from the diff, and even though I bought a 4oz bottel of the Redline posi additive, I have not had to add any additive to the diff at all. I've taken the car to a large parking lot and done lots of tight figure 8 turns and have ZERO issues with diff chatter or any other signs of issues. The Red Line gear oil worked great right out of the bottle.

And, call me crazy. . .the car seems a bit more responsive to the peddle.

Because of this positive experience, I fully plan on switching over to the 75w90NS oil in my muncie this summer.

WORD OF ADVICE, to anyone who is thinking about changing their gear oil. I was in the process of restoring the rear of my car, Trailing arms, bushings, suspension. . .the works. . .so I had removed the diff in the process. I have a nice workspace in my basement. . .and I decieded that I would do the diff restore in the basement. . .as it was much warmer than my 20 degree garage in the winter. WARNING, do NOT drain or add gear oil in your house. This stuff STINKS to high heaven!!!! My whole hose stunk for a week, and my wife was pissed! Do yourself a favor, and drain and add the oil outside.

 

[muddywaters]

Yep, Smells like dead fish..............