Now that Castrol has discontinued the long recommended 80W90 Hypoy GL-4 gear oil, Now what oil do we use in our Pantera Transaxles and why?

There are many discussions about what is the proper oil to use in the Pantera ZF 5DS-25/2 transaxle. Basically today there are 2 oils, a Castrol 80W90 LS (Limited Slip) and the Redline 75/90 NS, available from Pantera Parts Connection (1-800-DeTomaso (338-6627)) now, so which should we choose?

The answer is complicated because The ZF gearbox is not just a simple Transmission Gear only type design so a lubricant does indeed need to satisfy the requirements of a limited slip differential ... plus the ring and pinion gears, bearings of all types, transmission gears and the synchronizer rings. The lubricant must also be non-corrosive to steel, aluminum, and "yellow' metals (i.e. copper, brass or bronze) especially if your ZF has these brass synchro rings.

ZF's recommended viscosity was SAE 80 n 1972 or SAE 80W90 stated in today's modern lubricant technology. The synchronizers, brass/bronze parts and limited slip differential within the gearbox has traditionally meant lubricants rated API GL-4 are the best lubricants for the ZF. GL-4 lubricants are medium load lubricants in which the additive package has been specifically formulated to meet the requirements of limited slip differentials and synchronizer rings, while being non-corrosive to aluminum and yellow metals. API GL-4 lubricants were common lubricants for transmissions and transaxles. The Castrol 80W90 Hypoy Gear Oil (GL-4) was among the most favored lubricants ZF originally recommended to Ford but Castrol has now discontinued this GL-4 grade since 2008. Now what?

RBT states that they recommend that you use an API GL-4 rated gear oil. The RBT/ZF transaxle designs use the highly efficient Spiral Bevel style crown wheel and pinion set. There is no Hypoid offset in this type of gearing; hence, there is no longitudinal sliding along the face width of the tooth. Therefore, the additional friction modifiers used in GL-5 type oils are not required. These friction modifiers, required to reduce heat and prevent scoring on Hypoid gear sets, can have an adverse effect on the function of the transaxle synchronizers, for example, requiring greater shift effort. But all the oils available today are GL-5 rated so they need to be specially blended and balanced with friction modifiers.

Today if you have Brass Syncro Rings installed in your ZF we must recommend to put you into a newer formulation Friction Modified Synthetic like a RedLine 75W90 NS GL-5 LS, available from the Pantera Parts Connection, for use in differentials with the Spiral Bevel style crown wheel and pinion set found in the ZF or RBT Transaxle. This oil is designed to help slow synchro's for easier shifting across a broad temperature range. It contains the extra pressure additives, but lacks friction modifiers to balance slipperiness. This oil incorporates low sulfur content for Improved copper corrosion protection to prolong synchro life. This Synthetic oil helps with Limited Slip lock up and can be additionally tuned with Redline limited slip Friction Modifier if limited slip clutch chatter becomes evident.

Standard GL-5 lubricants found in most auto parts stores have a higher load rating than GL-4 lubricants, but the GL-5 additive package is usually not compatible with limited slip differentials or Brass synchronizer rings; the GL-5 additive package is usually corrosive to other yellow metals too. However, some API GL-5 lubricants contain friction modifying additives specifically added for limited slip differential usage (the label should specify it has a modified API rating such as GL-5+ or GL-5-LS). API GL-5 lubricants are common lubricants for axles that are independent from the transmission. Even when modified for limited slip differentials GL-5 lubricants are not usually considered suitable for the synchronizer rings & yellow metals in a transmission or transaxle. So use of these standard GL-5 oils is not recommended for long term use.

However, in the case of Castrol's current non synthetic 80W90 Hypoy Gear Oil (which is rated API GL-5 LS) available from Pantera Parts Connection, which has been Transaxle builder RBT's favored recommendation which indicates to me it must be OK for the ZF's for half the cost of the Redline synthetic oil, at least when using the RBT Cast Iron Synchro's that RBT Sells and uses in their ZF/RBT rebuilds. Beware because ZF also produces Brass Synchro's for the ZF-5DS-25/2 that some rebuilders use that are less expensive and will work fine as long as you use the Synthetic Redline oil described above.
That is why every question is answered and qualified with another question. How fast do you want to go, and for how long?