The Oily Truth

By Arlin

Will your motor really make more power, get better fuel economy, last longer, and run cooler, just by switching to a different brand of oil?

I’ve read those claims on the back of a few synthetic oils, and always wanted to test them side by side. Well, last winter I got a call from one of the newer oil companies, wanting to know if I was interested in doing an oil shoot out on the dyno. They were claiming their oil would make more HP than any other brand. I said yes immediately, and they took my address to send me a case of oil for the test. It never showed up, but I got myself so charged up about doing a test like that, I decided to do my own test with four different brands of oil — the ones most H-D owners prefer.

The contestants are: Harley-Davidson Synthetic 20/50, Mobil One 15/50, Amsoil Synthetic 0/30, and Royal Purple 20/50.

I had a good idea of how to do the test for heat and HP, but no way to measure fuel economy so it would be fair. But my thinking here is, there are two ways of making HP. Better combustion, and reduced friction. If you reduce the friction, better economy is a given.

The bike I had in for a tune was an ’02 Softail. It was in for a re-map. I started with fresh oil and a filter change, using Harley-Davidson synthetic in the motor. I decided not to test the oil in the transmission and primary. I wanted to get results from one thing at a time. We’ll do the drive train down the road. For now, both primary and transmission received the same Harley-Davidson 20/50 Synthetic.

After warming up the motor I took a base line run, and tuned the motor for full throttle only. This gave me a base number to work from.




1. Let the motor warm up to 250 deg. Measured at the rear head just off the plug surface. This temp was measured with a heat gun, with the fans full on.


2. Once the temperature was reached, the initial tuning starts. I watched the oil temp rise as the tuning progressed. The first section of the tuning session brought the oil temp up to 200 deg.


3. Next was a full throttle pass to get a HP reading. Our test bike made just under 72 HP. We made 3 pulls to get a reliable number. Then we drained the oil, changed the filter, and filled the oil tank with Mobil One 15/50, contestant number 2.

My goal was to repeat the same test with each oil, going through the same process of warming the motor up. We brought the motor temperature up to 250 degrees then watched the oil temperature rise, while tuning the next section. It finally reached a high of 205 degrees after our 3 full throttle pulls, returning an average HP figure of 71.75 — right on par with the Harley Oil.

Once again, I started changing the oil and filter, all the while thinking I was wasting time and money doing this test. I had started to think none of the oils tested would show any gain. But, I had the oil and a bike to tune, so out came the old, and in went 3 quarts of the next contestant: Amsoil 0/30.

So far, the first two oils didn’t have any extravagant claims about more HP, mileage, and longer life on the side of the bottle. But the Amsoil had a whole pull-out pamphlet stuck to the side of the bottle, claiming cars can go 17,000 to 35,000 miles, before an oil change. And, that it’s the highest quality oil you can buy, designed for the heaviest loads, most heat, for both diesel and gas engines— also, including racing engines. I smiled as I put the oil in the tank, knowing damn well there wouldn’t be any more power.

I started the bike to let it warm up. First thing I noticed, I had to turn off the fans, and let the bike idle for what seemed like forever, before the engine temperature got to the 150 degree mark. This got my attention right away, since both of the other oils got up to a 200 degree temperature, within only a few minutes of idling.

I started tuning the next section, all the while watching the oil temp. It wasn’t moving, or at least it was moving very slow. I decided to go on with the tune, until the temp reached at least 200 like the rest had.

To my surprise, I ended up finishing the mapping session, and the oil temp never got past 180.

Next step was a full throttle pull — that always gets the oil temperature up. Finally, the Amsoil topped out at 195 degrees. I thought the thinner oil would run hotter than the thicker stuff, but it seems to be just the opposite. When I saw the HP graph, I couldn’t believe it. 73.4 HP! I thought it was a mistake, so I made a second run — HP 74.2! Just one more to be sure — 74 HP. Wow, that was interesting. It really did make a difference. Oh, one other thing I almost forgot about. When I got the first run at 73.4, in looking at the graph for air/fuel ratio, I noticed the ratio had changed. The motor had gone rich, moving from a perfect 13 to 1, down to 12 to 1. Why? I don’t know. I’ve thought about it, but can’t explain why. All I know, is I adjusted the air/fuel ratio back to 13 to 1, by leaning the mixture out. Wouldn’t that give you more fuel mileage, since you’re using less fuel to make more power with the same 13 to 1 ratio? The next two runs after making the adjustment, averaged 74 HP!

Bottom line: we gained 2 real HP with the Amsoil, and more than likely better fuel economy.

While I drained the oil, I had high hopes for the last contestant, since it was the one that had all the HP, better mileage and wear claims – Royal Purple 20/50.

In all fairness, all the oil comparisons are between petroleum oil and their synthetic. I didn’t do this kind of test, but I’m sure after what I saw, synthetic oil outperforms petroleum oils, with ease.

Warm up was much like the first two. I ran the bike until the engine temp hit 250 at the head, all the while watching the oil temp, as it came up to just under 200 deg. Then, a final full throttle pass, for a HP reading of 72. Oil Temp topped out at 200 degrees. Air/fuel ratio had to be set back at the original setting, to obtain the same 13 to 1 mixture. No increase in HP after 3 full pulls.

The winner for sure, with less operating temp and more HP, is the Amsoil — winning with 195 deg. and 74 HP. I was impressed to say the least! The big question is, was the gain because of the lighter viscosity?

I can’t give you a good answer. After calling Mobil, Royal Purple, and Amsoil, I never got a good clear answer about the viscosity issue. They would only quote me their recommended oil viscosity for air-cooled Harleys. Yet their racing oil is a much lighter weight. Plus, all the major auto manufacturers are running lighter weight oils, for more power and better mileage. I was told the clearances are much tighter in the water cooled engines, than the air cooled Harley engine. So far, I haven’t been able to confirm this. The only real information I could get about viscosity, is that the big number in a multiple viscosity rating is the real weight of the oil, while the small number is the temperature the oil will pour out of the bottle.

But the big deal is, the fact that the super high grade synthetics hold their viscosity under heavier loads and greater heat, much longer than poorer quality oils, both petroleum and synthetic. The light stuff is even recommended for heavy duty diesel engines — but not motorcycle engines. After asking the same question from 3 of these oil manufacturers, I still couldn’t get a straight answer about why not use a lighter viscosity oil?

If you have a good answer to this question, please let me know at

One thing for sure, free HP isn’t easy to find, and usually costs an arm and a leg for every fraction of power. All these synthetics cost about the same, but for sure don’t all produce the same results. The 0/30 Amsoil, ran cooler than the other oils, and consistently made 2 more real HP, while maintaining a cooler engine temperature.

One more thing I can’t stop thinking about – the label on the Amsoil bottle. If it’s really possible to go 35,000 miles and more, without an oil change, is someone brain washing us to change every 3,000 miles? Some think it’s the oil companies. Not so. The oil companies are the ones recommending the ultra long intervals between changes. Here’s something to think about.

There are lots of vehicles out there. Let’s take a small number like 100,000. Average mileage per year for a car is 12,000 Oil changes every 3000 miles at 6 quarts each = 24 quarts, or 6 gallons a year per car Now multiply that by 100,000 cars. WOW, that’s 600,000 gallons every year. Can you imagine how much would be saved in this country if every vehicle went to the oil manufacturer’s recommended schedule?

Not only would we be saving oil and money, after these test results, it looks like we would be making more HP, while using less fuel. Who do you think knows the most about these oil products? Wouldn’t you think they (the manufacturers) would give us the correct information? For me, it goes against everything I ever knew or thought I knew about oil. But, after looking at the test results, and reading the information about synthetic oil, it’s got me thinking. Until I can get a better answer about the viscosity issue, I’m using the Amsoil 20/50 motorcycle oil, as recommended by the manufacturer. But, I’ll be waiting for the day when H-D starts recommending less viscosity oils. Remember what they recommended a few years ago for the transmission?

Heat in an air cooled motor is a big problem. It can cause detonation, and pre-mature wear on parts. Three things I firmly believe after doing the test — all bikes, from stock to fully modified, need an oil cooler, oil temperature gauge, and should be running synthetic oil with a high quality oil filter. Modified bikes with bigger motors, really need to run 2 oil coolers, as well as an octane booster.

I had a chance to do a test on an octane booster, made by Amsoil. If you have a big motor that’s been modified, you should for sure be running an octane booster. After putting 1/3 of a bottle in a full tank, I can assure you it will make more HP, especially when you’re running a poorer quality fuel. The bike I did this test on was consistently making 69 to 70 HP. I dumped the stuff in the tank, and gave it a minute to get in the system. After a few minutes of running, I took another dyno run. 75 HP was the best number, with 2 other runs at 74.5. Not only did it give us a very good gain in HP, but it allows the bike to run cooler, reducing heat in the cylinder head, which in turn reduces the over-all temperature of the motor and oil.

Cost of the octane booster is around $8 to $10. This will make 16 gallons. It’s great insurance for your motor – combining the increase from the oil, with the HP gain from the octane booster. And, you’re talking 6 or 7 HP. All of the synthetic oils cost very close to the same. Only added expense is the octane booster.