The major difference between API GL-4 and GL-5 oils is the amount of
sulfur and phosphorus (extreme-pressure) additives. GL-5 oils are formulated
with higher additive levels in order to adequately protect hypoid gears. You are
correct that the main concern with using GL-5 oils in transmissions is that the EP
additives will chemically attack yellow metals like brass synchro's. This certainly
may happen if the oil is poor quality.
AMSOIL Severe Gear 75W-90, however, is premium quality and easily meets the more
severe copper corrosion requirements of the manual transmission specification API
Note: a Corvair transaxle in a dune buggy most likely experiences much more
severe shock-loading than it would in a typical classic car. Under these conditions,
Severe Gear 75W-90 may provide even better protection than a GL-4 product such
as AMSOIL Synthetic Manual Transmission and Transaxle Gear Lube (MTG).
Preferred Customers purchase AMSOIL products at Dealer cost, without the responsibilities of owning a Dealership. AMSOIL also offers commercial and retail store opportunity/discounts too.
Independent Dealer since 2009 Sturtevant, WI
Order on-line from www.motorsportoilusa.com (My Amsoil Dealer Site)
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