Interesting information, apparently GM Synchromesh = Penzoil = Quaker State = Texaco = Mopar and all should meet MS-9224 whether it says so or not (from some other reading):
The information presented here is gathered from the documents listed in the References section below.
Pennzoil-Quaker State Synchromesh is a unique transmission and transaxle fluid that is not API "GL" rated. It has an approximate gear oil grade of about SAE 80W and can be used when temperatures range from -40ºF (-40ºC) to 300ºF (150ºC). According to Pennzoil's product description it is "formulated with high quality paraffinic base stocks, a fluidity modifier, multifunctional performance additives, corrosion inhibitors, a foam suppressor and a shear stable viscosity index improver additive. It provides excellent oxidation stability, low temperature performance, excellent synchronizer performance and compatibility with yellow metals, such as bronze, brass and copper components found in manual transaxles and transmissions." This description makes Synchromesh suitable for use in some transaxles or transmissions where GL-4 gear oils are specified.
A post on the message board at Machinery Lubrication Magazine Home
found Synchromesh to consist of a 5W30 non-detergent oil with a friction modifer and an EP (extreme pressure) additive. It is made under contract for General Motors by Pennzoil (APC 3378) or by Texaco (Code 1874 MTX Fluid).
According to Don Johnson, Synchromesh fluid was developed for use in the New Venture manual transmission used by Chrysler and General Motors. Specifically, Synchromesh is formulated for transmissions that require GM Part No. 12345349 (Specification No. 9985648), GM Part No. 12345577, or Chrysler Part No. 4874464 (Specification MS-9224). Owners of Diamond Star Motors (DSM) Eclipse, Talon, and Laser cars with MMC (Mitsubishi Motor Corporation) transaxles have reported positive experiences using Synchromesh. The same can be said for owners of the Mitsubishi 3000GT and Dodge Stealth cars with the Getrag-manufactured AWD transaxles. The main benefit to DSM and 3/S owners seems to be excellent synchronizer performance, with reduced grinding and easier shifting compared to using some other gear oils.
Pennzoil Synchromesh Fluid can be purchased as that brand or as the equivalent products: Quaker State Synchromesh (Quaker State is part of Pennzoil), Texaco Synchromesh, GM Synchromesh, or AC Delco Synchromesh (part number 89021808). GM Synchromesh retails for about $10 per quart bottle at GM dealerships. Pennzoil Synchromesh is sold at many Auto Zone auto parts stores for about $5 per quart and should be available from Pennzoil distributors. Texaco Express Lube stores use Synchromesh but the stores I contacted would not sell the product by itself to customers. Pennzoil Synchromesh can be purchased in 5-gallon containers from oil-store.com.