VISCOSITY INDEX is an empirical number indicating the rate of change in viscosity of an oil within a given temperature range. Higher numbers indicate a low change, lower numbers indicate a relatively large change.

FLASH POINT is the temperature at which an oil gives off vapors that can be ignited with a flame held over the oil. The lower the flash point the greater tendency for the oil to suffer vaporization loss at high temps and to burn off on hot cylinder walls and pistons. (degrees F.)

POUR POINT is 5 degrees F above the point at which a chilled oil shows no movement at the surface for 5 seconds when inclined. This measurement is especially important for oils used in the winter. The lower the pour point the better. (degrees F.)

% SULFATED ASH is how much solid material is left when the oil burns. A high ash content will tend to form more sludge and deposits in the engine. Low ash content also seems to promote long valve life. Look for oils with a low ash content.

% ZINC is the amount of zinc used as an extreme pressure, anti-wear additive. The zinc is only used when there is actual metal to metal contact in the engine. Hopefully the oil will do its job and this will rarely occur, but if it does, the zinc compounds react with the metal to prevent scuffing and wear. A level of .11% is enough to protect an automobile engine for the extended oil drain interval, under normal use. High zinc content can lead to deposit formation and plug fouling.

The Data: Listed alphabetically by weight

  Brand                     VI    Flash    Pour    %Ash   %Zinc
  20W-50
    AMSOIL (new)           157     507     -44     ---     ---
    Castrol GTX            122     440     -15     .85     .12
    Exxon High Performance 119     419     -13     .70     .11
    Havoline Formula 3     125     465     -30     1.0     ---
    Kendall GT-1           129     390     -25     1.0     .16
    Pennzoil GT Perf.      120     460     -10     .9      ---
    Quaker State Dlx.      155     430     -25     .9      ---
    Red Line               150     503     -49     ---     ---
    Shell Truck Guard      130     450     -15     1.0     .15
    Spectro Golden 4       174     440     -35     ---     .15
    Spectro Golden M.G.    174     440     -35     ---     .13
    Unocal                 121     432     -11     .74     .12
    Valvoline All Climate  125     430     -10     1.0     .11
    Valvoline Turbo        140     440     -10     .99     .13
    Valvoline Race         140     425     -10     1.2     .20
    Valvoline Synthetic    146     465     -40     <1.5    .12

  15W-40
    AMSOIL (new)           164      462     -49     ---     ---
    Castrol                134      415     -15     1.3     .14
    Chevron Delo 400       136      421     -27     1.0     ---
    Exxon XD3              ---      417     -11     .9      .14
    Exxon XD3 Extra        135      399     -11     .95     .13
    Kendall GT-1           135      410     -25     1.0     .16
    Mystic JT8             142      440     -20     1.7     .15
    Red Line               149      495     -40     ---     ---
    Shell Rotella w/XLA    146      410     -25     1.0     .13
    Valvoline All Fleet    140      ---     -10     1.0     .15
    Valvoline Turbo        140      420     -10     .99     .13

  10W-30
    AMSOIL (new)           162      520     -76     ---     ---
    Castrol GTX            140      415     -33     .85     .12
    Chevron Supreme        150      401     -26     .96     .11
    Exxon Superflo Hi Perf 135      392     -22     .70     .11
    Exxon Superflo Supreme 133      400     -31     .85     .13
    Havoline Formula 3     139      430     -30     1.0     ---
    Kendall GT-1           139      390     -25     1.0     .16
    Mobil 1                160      450     -65     ---     ---
    Pennzoil PLZ Turbo     140      410     -27     1.0     ---
    Quaker State           156      410     -30     .9      ---
    Red Line               139      475     -40     ---     ---
    Shell Fire and Ice     155      410     -35     .9      .12
    Shell Super 2000       155      410     -35     1.0     .13
    Spectro Golden M.G.    175      405     -40     ---     ---
    Unocal Super           153      428     -33     .92     .12
    Valvoline All Climate  130      410     -26     1.0     .11
    Valvoline Turbo        135      410     -26     .99     .13
    Valvoline Race         130      410     -26     1.2     .20
    Valvoline Synthetic    140      450     -40     <1.5    .12

  5W-30
    AMSOIL (new)           186      464     -76     ---     ---
    Castrol GTX            156      400     -35     .80     .12
    Chevron Supreme        202?     354     -46     .96     .11
    Chevron Supreme Synt.  165      446     -72     1.1     .12
    Exxon Superflow HP     148      392     -22     .70     .11
    Havoline Formula 3     158      420     -40     1.0     ---
    Mobil 1                165      445     -65     ---     ---
    Mystic JT8             161      390     -25     .95     .1
    Quaker State           165      405     -35     .9      ---
    Red Line               151      455     -49     ---     ---
    Shell Fire and Ice     167      405     -35     .9      .12
    Unocal                 151      414     -33     .81     .12
    Valvoline All Climate  135      405     -40     1.0     .11
    Valvoline Turbo        158      405     -40     .99     .13
    Valvoline Synthetic    160      435     -40     <1.5    .12

The numbers above are not, by any means, all there is to determining what makes a top quality oil. The exact base stock used, the type, quality, and quantity of additives used are very important. The given data combined with the manufacturer's claims, your personal experience, and the reputation of the oil among others who use it should help you make an informed choice.

Partial extract of article by: Ed Hackett ([email protected])