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When a maintenance technician finishes adding a charge of oil

to a critical hydraulic system, (s)he is not likely thinking about

the oil's individual components. Rather, his/her thoughts might

focus on how the oil will perform in the system. Will it lubricate

adequately? Will it cause deposits to form in the system? Will it

help keep contaminants from causing equipment damage and

filter plugging? Will it keep my equipment running smoothly

until my next scheduled maintenance interval?

If performance is what really matters, then why would anyone

spend time talking about formulation components? That's a

good question, but the practical reality is that there is a lot

of questions, concerns, and in some cases misinformation,

about the use of API Group I and Group II base stocks in

hydraulic oil formulations.

Base Stock Basics

Figure 1 shows the American Petroleum Institute (API)

categories for base stocks. These categories are based on

a few chemical and physical properties of the base stock.

Each property can be attributed to certain natural features ofbase oils. VI, viscosity index, is important when considering

applications exposed to temperature extremes. Sulfur provides

natural anti-wear protection. Saturates content, or chemical

composition purity, relates to oxidation stability and seal

compatibility. Not shown in Figure 1, but equally important,

is aromatic content. Aromatics, which are removed to

achieve higher saturates content, are the chemical

constituents important when considering a base stock's

natural solvency. Looking at Group I and Group II base

stocks only, Figure 2 shows which base stock naturally

exhibits the strongest features.

Group I vs Group II - Affect on Hydraulic OilsLow Temperature Capability

The low temperature properties of mineral oils are mostly

dependent on the dewaxing process used to finish the oils.

Although different dewaxing processes are usually used for

Group I and Group II oils, both can be formulated easily to meet

the low-temperature needs of most hydraulic applications.

Wear Protection

A key performance requirement for hydraulic systems is good

wear protection.Although Group I oils have an inherent level of

wear protection due to sulfur containing molecules in the oil

API Base Stock ClassificationPhysical Specifications

Group VI Sulfur

Wt. %

Saturate

Wt. %

ManufacturingProcess

I 80- 12 0 > 0.03 90

III >120 90

IV 14 0

typ

0.00

typ

>90 typ

V

Figure 1

Conventional

(Solvent Refining)

RequiresHydrocracking/Dewax

Requires Severe

Hydrocracking/Dewax

Chemical Synthesis

PAO

All other types-ester, polyglycols,phosphate esters...

Let's Talk Base Stock

Group I versus group II in hydraulic applications

Figure 2

Low Temperature Capability

Parameter

Comparison of base StockProperties

Group I Group II

Natural Wear Protection

Oxidation Stability (with AO)

Seal Compatibility

Additive Solvency

Contaminant/DepositSolvency

(removed from Group II), good anti-wear additives are available

which give most new hydraulic oils adequate wear protection

for today's demanding applications. As shown in Figure 3, wear

metals only accounted for 5% of used oil alerts for ExxonMobil

oils. However, an important question is how long will an oil

maintain its wear protection in use? For oils such as Mobil DTE

20 and Mobil DTE Excel, which have exceptional contamination

control and keep clean performance - leading to longer service

life, being able to maintain the anti-wear performance for

extended periods of time is a necessity and a key performance

advantage of these hydraulic oils. Most competitive products,

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whether formulated with Group I or Group II base oils, may not

require the extended wear protection since they will likely be

removed from service sooner due to a lack of contamination

control and keep clean (system cleanliness) performance.

Oxidation Stability

Although Group II oils are more oxidatively stable than Group I,

this difference only becomes significant at higher temperatures.

For instance, the ASTM D943 oxidation test, run at 95C

(203F), tends to give better results for Group II oils than Group I

(see Tech Topic "Burnt TOST" for discussion on relevance of

D943 to hydraulic oils). However, most hydraulic systems run

much lower in temperature, typically in the range 50-60C

(122-140F). In fact, based on nearly 50,000 ExxonMobil used

hydraulic oil samples, which upon testing generated alerts

identifying them to be either borderline or unsuited for further

service, only 2.5% were related to oxidation (Figure 3). None

of the alerts were due to oxidation alone.The bottom line is that,

in terms of practical benefits, the oxidation stability of Group I

versus Group II base oils is irrelevant for the vast majority of

hydraulic system operating conditions.

Solvency

The aromatics in Group I base oils give them excellent solvency

properties. This natural solvency and the right mix of performance

additives help to keep degradation and contamination products

from forming varnishes and deposits in operation.This is

especially important for modern systems with tight tolerances.

This natural solvency is removed from Group II oils to gain

better oxidation stability. Additional components can be added

to improve the solvency of Group II base oils. However, the

added expense often precludes this approach for hydraulic

oils. Between better oxidation stability or better solvency

(referring to Figure 3), where would you choose to have thebalance lie for your hydraulic fluid?

Seal Compatibility

Most people who have worked around hydraulic equipment

have seen the mess created when a system leaks oil (Figure 4).

This can occur in any part of the system, such as seals and

hoses, where elastomers are used. Since the hydraulic oilscome into direct contact with the seals and hoses, it is important

to understand any interactions that may occur between them. In

general, elastomer seals rely on the oil to swell them to create a

better seal with the equipment. Aromatic compounds in oil tend

to transfer into many common elastomer materials better than

saturated hydrocarbons. Therefore, in these cases, hydraulic oils

which use Group I oils tend to swell seals more than those which

use Group II oils. Similar to solvency, additional components can

be added to increase the seal swell with Group II oils. However,

the additional expense often precludes this approach for

hydraulic oils. Figure 5 shows ISO 1817 elastomer swell data

for two identical hydraulic oil formulations which differ only by

the base oil (Group I vs Group II).The test was run at the

standard test temperature of 100C and the test was extended

past the normal one week duration to nine weeks.The figure

shows that the Group I based formulation showed consistently

higher elastomer swelling over the entire extended test period.

Conclusion

Hydraulic oils can be successfully formulated with either Group I

or Group II base oils. Group I oils have the advantages of higher

natural solvency, which can improve contamination/deposit

control and additive solvency; natural wear protection; and

enhanced elastomer compatibility, which can help reduce

system leaks. Group II oils have a higher oxidation stability

and lower volatility, but neither of these attributes translates

into improved performance in typical hydraulic systems.

2005 Exxon Mobil Corporation. All rights reserved.

MOBIL, the MOBIL Logo (with red O), the flying horse device, EXXONMOBIL, and theExxonMobil Logotype are trademarks of Exxon Mobil Corporation or one of its subsidiaries.Mobil DTE and other brands are trademarks and brand names of Exxon Mobil Corporation or

one of its subsidiaries.

www.mobil.com

XT-0001SH

Oxidation + Others

Contamination

Wear Metals

92.5%

2.5%5.0%

ExxonMobil Used Hydraulic Oil Alerts

Figure 3

Based on 12 Years of Used Hydraulic Oil Data(49,389 Data Points)

Figure 4NBR Seal Swell

0

1

2

3

4

5

6

7

8

9

10

Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 Week 7 Week 8 Week 9

Group I Based Hydraulic Oil

Group II Based Hydraulic Oil

Figure 5