ISO VG 220 vs 320 Gear Oil: Which One Your Gearbox Needs

How do ISO VG 220 and ISO VG 320 gear oils differ?

ISO VG 220 and ISO VG 320 gear oils differ in viscosity class: the ISO VG 320 midpoint is about 45% higher at 40 °C, and each grade allows plus or minus 10% around its midpoint (ISO 3448:1992). The grade number says nothing about additives, base oil or load capacity. Those come from the oil type, such as an antiscuff or inhibited oil under ANSI/AGMA 9005-F16.

The table below decides which grade the published selection guidance points to for each operating condition.

The key row is the slowest speed band. For a mineral oil with a viscosity index of 90 at 1.0 to 2.5 m/s, the superseded E02 tables list ISO VG 220 at 45 °C and ISO VG 320 at 50 °C (STLE TLT, 2008), and ANSI/AGMA 9005-F16 also lists ISO VG 320 at 50 °C (Gear Technology, 2018). Above 2.5 m/s, the E02 tables list ISO VG 100 or lower up to 50 °C (STLE TLT, 2008), and ANSI/AGMA 9005-F16 gives ISO VG 150 at 5 m/s and 50 °C (MPMA RFI 23-1, 2023).

When is ISO VG 220 gear oil the better choice?

ISO VG 220 gear oil is the better choice when the gearbox nameplate names it, or, without a maker grade, when the gears run faster or the oil runs cooler than the conditions that call for ISO VG 320. Siemens states for its Flender H, B and T series units that the type, quantity and viscosity of the oil are given on the rating plate (Flender BA 5010, 2010).

Published tables point to ISO VG 220 in a narrow window. At 1.0 to 2.5 m/s pitch line velocity, the AGMA 9005-E02 tables reproduced by STLE list ISO VG 220 at 45 °C oil operating temperature for mineral oils and polyalphaolefin (PAO), and for a polyalkylene glycol (PAG) oil even at 50 °C (STLE TLT, 2008). Faster gears need less: high velocities are generally associated with light loads and very short contact times, and low viscosity oils are usually adequate there (USACE EM 1110-2-1424, 2016).

ISO VG 220 also helps where the oil must move through pumps, filters and coolers on a cold morning, because a thinner oil flows more easily. Robert Errichello advises considering criteria affected by oil viscosity such as pumpability, efficiency and operating temperature (Gear Technology, 2018).

When is ISO VG 320 gear oil the better choice?

ISO VG 320 gear oil is the better choice when the nameplate names it, or, without a maker grade, when the slowest gear mesh runs at low pitch line velocity with the oil at about 50 °C or warmer. ANSI/AGMA 9005-F16 Table B.1, for a mineral oil with a viscosity index of 90 at 1.0 to 2.5 m/s, lists ISO VG 320 at 50 °C operating temperature, ISO VG 460 at 55 to 60 °C and ISO VG 680 at 65 °C (Gear Technology, 2018). Operating temperature in that table means the bulk oil temperature supplied to the gear teeth (Gear Technology, 2018).

Low speed and heavy load push toward the thicker grade. The US Army Corps of Engineers (USACE) states that low speeds are associated with high loads and long contact times, which require higher viscosity oils (USACE EM 1110-2-1424, 2016). It advises selecting gear oils for the highest viscosity consistent with the operating conditions (USACE EM 1110-2-1424, 2016).

Surface fatigue is the other reason. ANSI/AGMA 1010-F14, as cited by Errichello, recommends an adequate amount of cool, clean and dry lubricant of adequate viscosity to reduce the risk of macropitting (Gear Technology, 2018). When several gear sets share one sump, evaluate the lowest speed mesh (STLE TLT, 2008).

Can you use ISO VG 320 instead of 220 in a gearbox?

You can use ISO VG 320 instead of ISO VG 220 in a gearbox only when the gearbox maker approves it for that unit; otherwise keep the nameplate grade. ANSI/AGMA 9005-F16 states that it is not intended to supplant any specific recommendations of gear manufacturers (AGMA, 2026), and its viscosity tables are offered only in the absence of maker recommendations, application standards or a rigorous elastohydrodynamic (EHL) analysis (MPMA RFI 23-1, 2023).

The thicker grade is not a free safety margin. USACE states that an oil viscosity that is too high results in excess friction and in degradation of oil properties associated with high oil operating temperature (USACE EM 1110-2-1424, 2016). A higher grade also changes start-up: an ISO VG 320 can have a higher pour point than an ISO VG 220 from the same product line (Petro-Canada, accessed 2026), and pumps, filters and spray nozzles may have been sized for the original grade.

Before a grade change, ask the maker whether the bearings, seals and circulation system accept the new grade, and whether a warranty depends on the original grade.

Will ISO VG 220 gear oil run cooler than 320?

ISO VG 220 gear oil usually lets a splash-lubricated gearbox run somewhat cooler than ISO VG 320, provided the thinner oil still separates the teeth. The reason is fluid friction. USACE lists oils that are too viscous for the application among the causes of high operating temperature, with excess oil in the housing and overload (USACE EM 1110-2-1424, 2016).

The effect has a limit. If the thinner grade is too thin for the load and speed, the film collapses into mixed lubrication, and USACE notes that selecting too low a viscosity can result in mixed lubrication, where asperities contact and wear increases (USACE EM 1110-2-1424, 2016). Metal contact then adds friction heat, so a grade below the maker's grade does not reliably lower temperature.

The sources reviewed give no general figure for the temperature drop from ISO VG 320 to ISO VG 220; it depends on speed, immersion depth and housing design. Measure it: log sump temperature at the same load before and after a maker-approved change.

Which is better in cold weather, ISO VG 220 or 320 gear oil?

For cold weather, neither ISO VG 220 nor ISO VG 320 is better by grade alone: keep the grade the gears need at operating temperature, and solve the cold start with pour point, heating or a synthetic oil. USACE states that when very low ambient temperatures are encountered, the oil viscosity should not be lowered, because a reduced viscosity may be too low when the gears reach normal operating temperature (USACE EM 1110-2-1424, 2016).

The start-up check is pour point. Gear oils should have a pour point at least 5 °C lower than the lowest expected temperature (USACE EM 1110-2-1424, 2016). Pour point varies by product more than by grade: one antiscuff (EP) gear oil line lists a pour point of minus 33 °C for ISO VG 220 and minus 21 °C for ISO VG 320 (Petro-Canada, accessed 2026). Read the data sheet of the oil you buy.

When the pour point is not low enough, heat the oil. Siemens notes that at low temperatures it may be necessary to heat the gear oil before switching on the drive unit or even during operation (Flender BA 5010, 2010). USACE names sized oil heaters or a synthetic oil compatible with the gear materials as alternatives (USACE EM 1110-2-1424, 2016).

What happens if you put ISO VG 220 in a gearbox that needs 320?

If you put ISO VG 220 gear oil in a gearbox that needs ISO VG 320, the oil film between the teeth gets thinner at every operating temperature, which raises the risk of mixed lubrication and wear. The illustrative calculation below uses one maker's data for an antiscuff (EP) ISO VG 220 (220 mm2/s at 40 °C, 19.4 mm2/s at 100 °C) and ISO VG 320 (321 mm2/s at 40 °C, 24.9 mm2/s at 100 °C) (Petro-Canada, accessed 2026). Viscosity at other temperatures follows the ASTM D341-20(2025) viscosity-temperature equation, and minimum film thickness scales with viscosity to the 0.68 power in the Hamrock and Dowson point contact formula (NASA TP 1342, 1978), at equal speed, load and pressure-viscosity behavior.

At 60 °C, the illustrative ISO VG 320 gives a film about 25% thicker than ISO VG 220. Put another way, ISO VG 220 at about 53 °C has the viscosity that ISO VG 320 has at 60 °C (illustrative). Dropping one grade costs roughly the film of running about 7 °C hotter.

How do you switch a gearbox between ISO VG 220 and 320?

You switch an industrial gearbox between ISO VG 220 and ISO VG 320 by getting the maker's approval, draining, flushing with the new oil and refilling to the mark. Siemens states that the gear unit must, if necessary, be flushed out with the new oil grade, unless the oil supplier confirms full compatibility (Flender BA 5010, 2010).

1. Stop the drive and secure it against switching on, under your energy control procedure, such as US OSHA 29 CFR 1910.147 (US OSHA, 29 CFR 1910.147).

2. Drain while the oil is warm. Siemens warns of a risk of scalding from escaping oil during oil changes (Flender BA 5010, 2010); wear the protection listed in the oil's safety data sheet (SDS).

3. Flush the housing and any circulation system with the new grade.

4. Collect used and flushing oil separately and manage it under the used oil rules where you operate, such as US EPA 40 CFR Part 279 (US EPA, eCFR as of 2026-10-08).

5. Refill to the sight glass or dipstick mark, which Siemens treats as decisive (Flender BA 5010, 2010), and record the new grade.

Lubinpla can work a grade question like this from your own documents, such as the oil TDS, SDS and gearbox records, and return one cited document for your review.