Should You Put Synthetic Oil in Your Older GM Truck?

Let's say you've got an 80s or 90s era GM pickup. And, let's say that you want to run synthetic in your engine. The benefits are well known - synthetic oil makes engines run a little smoother, lasts longer than conventional motor oil, and can offer more protection for your engine than conventional oil in specific situations (like towing and off-roading).

But, on the other hand, your 80s or 90s GM truck might not specify synthetic oil in the owner's manual. Is synthetic oil OK to use? Is it really going to work as promised, if the engine wasn't designed for it?

Here's what you need to know.

First, Synthetic Oil Isn't Going To Damage Your Truck's Engine

You may have heard that switching to synthetic oil in an older car will lead to engine damage. While this isn't correct, it used to have some truth to it. Decades ago, synthetic oil contained esters that could wear down the seals in an old engine and cause leaks. We no longer have to worry about this problem because synthetic oil technology has grown leaps and bounds since its ester-laden infancy.

Synth oil Photo credit: SIGAUS

However, it may be true that your older engine will consumer synthetic oil more quickly than conventional oil. Conventional motor oil molecules are often larger and more inconsistently sized than synthetic oil molecules. The uniform (and relatively small) size of synthetic oil molecules can lead to more oil slipping between the rings and the cylinder wall, increase oil leakage thru the valve guides, etc.

But - as long as you keep an eye on the consumption rate and oil levels - this increased consumption isn't going to cause any damage.

What You Might Like About Using Synthetic In Your Truck

Compared to conventional oil, synthetic oil has quite a few benefits.

  • Lasts longer - Synthetic can go 5,000 or even 10,000 miles between changes.
  • Cleansing benefits - Synthetic oil can sometimes clean oil system build-up out of an older engine. However, to be fair, there are conventional oils that do this too with the right forumlation.
  • Better viscosity in extreme temperatures - Synthetic flows better in very cold and very hot conditions
  • More breakdown resistance - Oxygen and heat are the primary "killers" of motor oil. Synthetic oil is more resistant to both, meaning it won't break down as easily as conventional motor oil after a long climb while towing, an off-road trail adventure, etc.
  • Improved fuel economy - Internal engine friction is a big drag on fuel economy, and synthetic oil reduces that drag. It's not uncommon for synthetic oil to boost fuel economy 2 or 3%.
  • Better engine protection - Generally speaking, synthetic offers better overall protection than conventional motor oil (only there are a lot of ifs and buts on this point)

In a nutshell, synthetic oil is more versatile than conventional oil. It performs better in abnormal situations, offers more protection, and improves fuel economy. For all these reasons, nearly every new car built today is filled with synthetic oil at the factory.

As for older truck engines, the benefits of synthetic motor oil are going to depend on your situation.

When It's a Good Idea To Use Synthetic

1. You don't drive much. Synthetic oil lasts much longer than conventional oil, as it doesn't oxygenate as quickly. If you've got an older truck that spends most of its time parked - and maybe drives 5,000 miles a year - synthetic is a good idea. Conventional motor oil loses a lot of its effectiveness after six months simply being exposed to the atmosphere. Synthetic oil lasts much longer.

2. You idle your truck a lot. Your truck needs air flow to keep the engine cool. If you’re idling it, you’re not providing it with any air flow and the engine can get pretty hot. Synthetic oil does a much better job at keeping your engine cool than conventional oil does.

NOTE: Stop and go driving involves a lot of idling.

3. You live in a desert or high altitude environment. Deserts have hot air. Areas at high altitude (over 10,000 feet) have thin air. Both environments lead to hot engines. Synthetic oil can offer more protection.

4. You live in a cold area. Synthetic oil has more viscosity at freezing (or even sub-zero) temperatures than conventional oil. This makes synthetic a great option for trucks that are parked in extreme cold.

5. You push your truck hard. Off-roading, racing, and towing are all strenuous activities as far as your engine is concerned. Synthetic offers more protection in these types of situations.

When Synthetic Oil Is Probably Overkill

1. Your truck is a commuter vehicle. If your truck is mostly hauling you from point A to point B, and doing so without a lot of traffic or time spent idling, it probably won't benefit from synthetic oil (in terms of protection). Only, if you commute a long ways, you might want the fuel economy benefits...

2. Your truck consumes a fair bit of oil. According to GM, 1-2 quarts of oil consumption between oil changes is normal. We think this is pretty high. If your truck's engine is consuming 2 quarts of oil every 3,000 or 5,000 miles, you're probably better off going with the cheap stuff. There's not much sense in spending top dollar on oil that's going out the tailpipe.

3. You've got an oil leak. Maybe it's just a seeping gasket, or maybe it's something more serious. But whatever it is, if you've got oil leaking out, you want conventional. It's going to leak more slowly.

Finally, Avoid Synthetic Blends

No discussion of synthetic or conventional oil is complete without talking about "synthetic blends."

While every synthetic blend is different, we generally don't recommend them because:

  1. Synthetic blends aren’t recommended by any vehicle manufacturer. It's either synthetic or conventional - not a mix of both.
  2. The "blend" can be 99% conventional oil and only 1% synthetic oil - there's no rule that says the blend has to be 50-50. In fact, many blends don't indicate the percentages for precisely this reason.

Summing up, synthetic oil is a good choice for a lot of vehicles, even older GM trucks. However, it's not for every truck. What's more, the high cost of synthetic might not be worthwhile (only, with the way motor oil prices have shot up in the last 10 years or so, who knows). But there's no risk to your engine to try it out and see how it goes. A lot of truck owners prefer synthetic just because their engine runs a little more smoothly, so give it a shot!