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Is it Synthetic?
Some scattered thoughts...
There are three basic categories of base oil on the streets. Group I and II are refined petroleum. Group III is refined petroleum that has also been dewaxed (improves low-temp performance) and high-pressure hydrogen processed to improve high temperature performance. Group III can be called 'synthetic' (legal definition) in the Americas but no where else in the world. Group IV is man-made synthetic base oil - PAO. Group V is an 'all others' group and contains petroleum and synthetic products - from Vaseline petroleum jelly to the man-made esters used as a component in lubricating oils.
Most of the products on the North American market - the 'synthetics' and 'synthetic blends' are made with Group III base stock. All of AMSOIL's products (with the exception of the XL products) is Group IV/PAO based. RedLine products are primarily ester based. (Both companies use a combination of PAO and ester.). Mobil 1 products are primarily PAO-based, but some also contain some amount of Group III (legally synthetic, but technically a synthetic blend because of the petroleum-derived content.)
Gear teeth are kept apart by a combination of fluid viscosity and the extreme pressure components in the oil's formula. Moving to a lower viscosity product can increase efficiency but can also result in increased wear rates.
As viscosity thins, the anti-wear additives must be improved to maintain the same low wear rates. Keeping wear in check is important because it's a geometric progression - one piece of steel or grain of sand in a gear box can scrape a pair of pieces from two gear teeth. The three pieces can generate 6 new pieces, the current 9 pieces can generate 18...and away we go.
It's best to match both the viscosity and the performance rating (such as GL-4) when selecting a replacement fluid.
If the transmission is filled with 75W-90 GL-4 petroleum, move to 75W-90 synthetic (AMSOIL, Redline, Mobil 1, or a European product) that also has the GL-4 (or similar OEM rating). Next move might be to something in the Synchromesh arena.
Driver technique and aerodynamics are better first targets - efficiency gains from lighter lubes are measurable but small, and if taken too far can lead to unhappy synchronizers and/or increased wear.
GL-5 is for differentials; GL-4 is for transmissions with 'yellow metals' and synchronizers. The higher levels of extreme pressure additives in GL-5 fluids can destroy brass, bronz, and copper in a manual transmission. Some 'real' synthetics can have dual rated GL-4/GL-5 products because the company found that the heat-activated GL-5 additives don't activate in a cooler running transmission. But it's best to stick with GL-4 or appropriate OEM rating for a synchronized transmission.
Please do not add anything to any finished lubricating oil - including transmission, engine, or differential. At best you're only out a bit of money. At worst one can lose their component warranty, oil warranty, have bearing wear and/or increased rust/corrosion. Lubricating oils are designed and tested specifically for the intended application - don't upset the sensitive chemical balance.
Andy
'97 VW Passat TDI wagon 391,000 miles
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