Can replacing the oil in a damper really improve performance?

Change Your Oil—No, Your Other Oil

One of the most persistent performance fables involves shock absorbers. Back in the day, racing rules required that Showroom Stock competitors retain their cars’ factory dampers. Racers being racers, they looked for a way to upgrade their suspensions while skirting the rules.

Can a stock shock absorber’s performance really be improved by swapping its oil for thicker stuff? And—perhaps more importantly—can this trick be used by today’s low-buck enthusiast as an inexpensive handling upgrade?

Why It’s Supposed to Work

The main purpose of shock absorbers is to damp the movement of a car’s suspension springs—without them, the springs would continue to oscillate up and down, causing the car to behave like a bobblehead doll. From the outside, a shock absorber looks like merely a simple piece of tubing sporting some protrusions and fittings. Inside each shock absorber, however, there’s a shaft-mounted piston moving through oil. Small holes drilled through the piston itself, as well as one-way valves, determine how much force it takes for the shaft to move through the oil—and, as a result, how much force the shock exerts in resistance to movement.

The fine art of shock absorber tuning involves getting those inner guts, including the oil, just right for the intended application. It’s generally a process left to the experts, but this speed tip is seductively simple: Just replace the original oil with higher-viscosity stuff and, in theory, the shaft will have to use more force to move. This added damping would, according to legend, provide increased composure through turns, under braking and during acceleration. Even a bone-stock car’s comfort-tuned shock absorbers would perform more like the high-dollar performance units. 

Could this stuff of rumors really provide a low-buck performance solution for today’s shoestring-budget racer? After all, 20 bucks’ worth of oil is a lot more palatable than a few hundred dollars for new struts. 

Our Test

We needed a test subject and found a convenient solution on our shelf of spares. Enter a pair of factory Toyota struts removed from the rear of a 1991 MR2 that had seen more than 100,000 miles of use. 

The struts were bolted to a Roehrig shock dynamometer and put through their paces. We found that despite their age and mileage, the struts functioned pretty well, generating around 300 Newtons of force at 0.1 meter/second of shaft movement. 

Next, it was time to start the transplant. We unscrewed the gland nut at the top of each strut body and dumped the factory fill into a graduated cylinder. Each strut held approximately 225-milliliters of fluid. The stock fluid looked thin—based on our experience, we’d guess that it was 5-weight. 

If doing this procedure on gas-charged units, take caution: Wrap a rag around the damper to contain the mess, and wear eye protection. Our friends in the industry warn against replacing the oil in gas-charged, monotube units. The procedure cannot be safely done at home, they stress.

We then replaced the oil with the same amount of 10-weight synthetic shock fluid from Maxima Racing Oils. We purchased a 500-milliliter container of fluid, which was more than enough to fill both of our test struts.

Refill the inner cylinder and then carefully reload the piston assembly to minimize air bubbles.

Back on the dyno, the 10-weight oil showed about a 10-percent increase in force over the stock fluid at 0.1 meter/second. While that was an improvement over stock, we admit to feeling disappointed. The viscosity of the fluid had doubled, yet the gain was only incremental. 

Our next step was to replace the 10-weight oil with a much more substantial 30-weight SVI suspension fluid from Rock Oil. We purchased the 500-milliliter bottle online.

Our Results

The dyno showed that this much heavier oil really did the trick, effectively doubling our damping force at the same shaft velocity. We were now registering around 600 Newtons at 0.1 meter/second and observing a much more digressive curve, with stiffer damping at lower shaft speeds. 

While our laboratory data is certainly encouraging, it does come with a few asterisks. First, the fact that our Toyota struts had a screw-on gland nut made the fluid swap unusually easy. Most struts and shock absorbers are welded together and require drilling or cutting if you want to drain the oil. Then, once you’ve replaced the fluid, you have to seal up the unit somehow. 

Another caveat: We didn’t replace the factory gas fill. That gas is often added to prevent the oil from foaming during severe use. In theory, losing the gas could hurt shock absorber performance during long races; but on the other hand, most high-performance synthetic shock absorber oils are resistant to foaming. 

Finally, at the end of our testing, we still had a pair of old struts featuring ancient shaft seals—and who knows how long those seals will last. There’s a chance that the thicker oil will blow out those fragile seals, rendering our rehabilitated dampers worthless. 

Take home message: If you’re trying to improve performance for just pocket change and navel lint, then changing your shocks’ oil may be the hot ticket for improved performance.