Sunday, 01 October, 2023

Best Sportster Primary Oil Choices


When choosing the best primary oil, there are a few things to consider. Read on to learn more than you ever wanted to know about choosing a primary oil for your Harley Davidson Sportster.

First, we have the OEM Primary Oil offerings:

Harley Davidson Formula+ is a conventional mineral oil with a cSt of about 165 at 40 degrees Celsius. That places it on the low end of the viscosity range of a 50w engine oil, or near the middle of the range for a 90w gear oil. It’s anti-wear additives include Zinc, Phosphorous, Molybdenum, Magnesium, Calcium, and Boron.

Screamin’ Eagle SYN3 is a synthetic blended (60%/40%) SAE 20w50 oil made by Cocono. It’s additives are “proprietary”, but boron, calcium, magnesium, phosphorous, and moly are present.

So, since both of the OEM primary oils are similar in weight (~SAE 50) and additives, it’s safe to conclude that similar 20w50 oils from Amsoil, Royal Purple, Mobil1, etc. would be a good choice.

Brand Indicated
Viscosity Grade
Measured Viscosity
@ 100° C cSt
SAE Viscosity
Range for
50 Grade
AMSOIL MCV 20w50 20.56 16.3 to < 21.9 Yes
Castrol V-Twin 18.49 Yes
Harley Davidson SYN 3 20.38 Yes
Honda HP4 17.58 Yes
Lucas High Performance 17.75 Yes
Maxima Maxum 4 Ultra 15.69 No
Mobil 1 V-Twin 21.04 Yes
Motul 7100 Ester 17.94 Yes
Pennzoil Motorcycle 20.69 Yes
Royal Purple Max-Cycle 20.09 Yes
Spectro, Platinum HD 19.26 Yes
Suzuki 4-Cycle V-Twin 19.82 Yes
Torco V-Series SS 21.05 Yes
Valvoline 4-Stroke 18.18 Yes


The results show that all of the oils tested except Maxima Maxum 4 Ultra have actual viscosities consistent with their labeling. So, they are good choices for your Sportster’s primary oil.

Best Sportster Primary Oil Choices: Narrowing the Field

An oil’s viscosity can be affected through normal use. Mechanical activity creates shearing forces that can cause an oil to thin out, reducing its load carrying ability. In our Sportsters, the primary oil is particularly subject to high shear rates due to being shared with the transmission.

After testing the shear stability of the oils above according to ASTM D-6278, nearly half dropped one grade to an SAE 40. Many of the oils losing a viscosity grade did so quickly, within the initial 15 cycles of shearing. So, we probably don’t want to use those.

The ones that stayed?

Amsoil MCV, Pennzoil Motorcycle, Mobil V-Twin, Torco V Series, Suzuki 4 Cycle, Spectro Platinum, Motul 7100, and Harley Davidson Syn3.

Notice that Formula+ is *not* on the list.

Primary Oil Testing: Wear

The ASTM D-4172 4-Ball Wear Test is a good measure of an oil’s ability to minimize wear in case of metal-to-metal contact. The test consists of a steel ball that sits atop three identical balls that have been placed in a triangular pattern and restrained from moving. All four balls are immersed in the test oil, which is heated and maintained at a constant temperature. The upper ball is then rotated and forced onto the lower three balls with a load measured in kilogram-force (kgf). After a one-hour period of constant load, speed and temperature, the lower three balls are inspected at the point of contact. Any wear will appear as a single scar on each of the lower balls. The diameter of the scar is measured on each of the lower balls and the results are reported as the average of the three scars, expressed in millimeters. The lower the average scar diameter, the better the anti-wear properties of the oil.

In this case, the load, speed and temperature used for the test were 40 kg, 1800 RPMs and 150° C.

Torco and AMSOIL motorcycle oils finished first and second, followed by Mobil 1 V-Twin, Maxima 4 Ultra, and Harley Syn3. Harley’s Syn3 was the only one of the top 5 to exceed 0.5mm scarring.

Since we want the *best*, we’ll ignore the oils that produced a larger scar.

Transmission Gear Wear Testing

Since our primary oil will also lubricate our sportster’s transmission, we want to make sure it’s protected, too! High sliding pressures, shock loading and the shearing forces applied by the gears demand a great deal from a lubricant.

To examine gear oil performance, the ASTM test methodology D-5182 (FZG) is used. In this test, two hardened steel spur gears are partially immersed in the oil to be tested. The oil is maintained at a constant 90° C and a predetermined load is placed on the pinion gear. The gears are then rotated at 1,450 RPM for 21,700 revolutions. Finally, the gears are inspected for scuffing (adhesive wear). If the total width of wear on the pinion gear teeth exceeds 20 mm, the test is ended. If less than 20 mm of wear is noted, additional load is placed on the pinion gear and the test is run for another 21,700 revolutions. Each time additional load is added, the test oil advances to a higher stage. The highest stage is 13. Results indicate the stage passed by each oil. Wear is reported for the stage at which the oil failed.

All of our remaining candidates passed all 13 testing stages.

Amsoil performed the best, with 0mm wear. Harley Syn3 performed the worst, with 14mm. The rest? A tie at 0.2mm wear. So, while Harley’s Syn3 *did* pass, I would only recommend using it if it’s the only one you can get when you need it.

AMSOIL MCV obtained consistently high marks in both the 4 Ball Wear and FZG testing phases. However, it’s typically something you need to order ahead of time and plan for since it’s not available in regular parts stores.

Mobil 1 V-Twin, however, scored very respectably in both testing phases and is available in most Oreilly’s, Autozone, and Walmart locations, among others, so it’s probably the Best Sportster Primary Oil for most people.

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