I spoke with a Valvoline tech about recommended rear gear oil. He said 80-90 GL5. Is this ok as I remember GL4 is recommended for Muncies.
Do I need any additive since it's an non posi ?
Thanks.

GL-4 for your manual transmission, and GL-5 for the differential. Actually, GL-4 is fine for non-posi differential applications. It has been claimed that GL-5 has compounds that might attack the bronze parts in a synchromesh transmission. My $0.02

FYI,HERES THE SUPPOSED GL5 BAD FOR MUNCIE SYNCRO,S AS STATED BY REDLINE OIL,NOT ME/LOL!!!!.

SCOTT
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REDLINE Synthetic Manual Transmission Lubricants

GEAR AND SYNCHRONIZER WEAR PROTECTION
Most manufacturers of manual transmissions and
transaxles recommend an 80W or 90W GL-4 lubricant.
GL-5 gears oils which are required in hypoid differentials
are not used in most synchromesh transmissions
because the chemicals used to provide the extreme
pressure protection can be corrosive to synchronizers,
which are commonly made of brass or bronze. Typically,
the use of a GL-5 lubricant in a synchromesh
transmission will shorten the synchronizer life by one
half. The extreme pressure requirements of spur gears
and helical gears found in transmissions are not nearly
as great as found in rear-wheel drive differentials. A
GL-4 lubricant provides adequate protection for most
manual transmissions, unless a unique design
consideration requires the extra protection of a GL-5.
The reason that many manufacturers have made
recommendations of motor oils or ATFs is that petroleum
80W gear oils frequently do not shift well at low
temperatures. Motor oils and ATFs are much more fluid
at lower temperatures and they are not corrosive toward
synchros, but they provide very poor gear protection.
These lubricants provide almost no extreme-pressure
protection. In addition, petroleum multigrade motor oils
and ATFs have very poor shear stability. The shearing
action by a manual transmission on thickeners is much
worse than in an engine or automatic transmission.
Within 5,000 miles the thickeners can be rendered
ineffective and the transmission will be operating on a
much reduced level of protection, as shown in the graph
below. In hot weather these transmissions will whine
and rattle because of poor vibration dampening and
metal contact. Red Line MTL and MT-90 provide the
excellent gear protection of a GL-4 gear oil in a synthetic
lubricant which spans hot and cold temperatures and will
not shear or oxidize with use.

DIRECTIONS FOR USE
Red Line MTL may be used in transmissions which
recommend 75W, 80W, or 85W GL-4 gear oils, or SAE
30 or 5W/10W30 motor oils. If a 90W GL-4 or SAE 40,
10W40, or 15W40 is required, MT-90 may be used. If the
transmission or transaxle requires an SAE 90 GL-5 gear
oil, then Red Line 75W90NS or 75W140NS Gear Oil
may be used. In transmissions which recommend Dexron
or Mercon fluids we recommend our D4 ATF which is
very similar to the MTL, being a GL-4 Gear Oil also. The
D4 ATF will provide better low-temperature shiftability,
and the MTL would provide better wear protection for
racing use. MTL is not designed for use in rear-wheel
drive differentials. Those generally require a GL-5 lubricant
such as Red Line 75W90 Gear Oil. It is not necessary
to flush the transmission before replacing with MTL.
Remove the drain plug and drain while warm. Seal
compatibility has been designed to be similar to petroleum
lubricants, and leakage should be no greater than
any other oil of comparable viscosity. Being formulated
with extremely stable synthetic basestocks, MTL and
MT-90 will last much longer than conventional petroleum
lubricants. However, we do not recommend extended
drain intervals, since without a filtration system, there is
no way to remove metal shavings other than draining the
lubricant. The regular maintenance intervals are also
recommended to insure that the proper level of the fluid
is maintained.


Red Line Oil's MTL and MT-90 are designed to provide excellent protection and improved shiftability for manual transmissions and transaxles, having cured the problem of hard shifting in thousands of transmissions with shifting troubles. How? They have the appropriate coefficient of friction for most manual transmission synchronizers (many gear oils, engine oils, and ATFs are too slippery for proper synchro engagement). And, the wide viscosity of MTL and MT-90 allow proper shifting over the entire temperature range which the transmission will experience. The synthetic base oils used have a very high viscosity index which provides relatively constant viscosity as temperature changes. MTL is a low 70W at very low temperatures and a high 80W, nearly an 85W, at elevated temperatures, providing adequate viscosity to prevent wear and deaden gear noise. MT-90 is a thicker 75W90 version of MTL. The shear stability and oxidation stability of these products are excellent, thus the physical characteristics of Red Line MTL and MT-90 will change little with use.

MTL
MT-90

The MT-90 is a 75W90 GL-4 Gear Oil that’s slightly heavier than MTL. Provides excellent protection of gears and synchronizers and its balanced slipperiness provides a perfect coefficient of friction, allowing easier shifting.


RED LINE SYNTHETIC OIL CORP.
6100 Egret Court
Benicia, CA 94510
(707) 745-6100
www.redlineoil.com

NOTE: AMSOIL ALSO OFFERS A 75W-90 SYNTHETIC GEAR LUBE FOR MUSCLE CAR/MUNCIE MAN TRANS.