There is always some doubt in what type of oil should be used in a 500, 126 and a126 BIS gearbox.

If you are not sure what gearbox you have a simple rule of thumb is, if the gearbox has synchromesh then it’s from a 126 (or 126 BIS) and if it’s a ‘crash box’ then it’s a 500!

Model	Fiat recommend	Equivalent
500 all models	TUTELA W 90/M	SAE 90 EP
126 all models inc. BIS	TUTELA ZC 90	SAE 90

EP means extreme pressure and refers to the additive in the oil. The additive is to stop metal-to-metal contact between the gears and is based on Sulphur/Phosphorous which forms a sacrificial chemical layer, thus protecting the metal. The sulphur gives EP gear oils their characteristic smell.

IMPORTANT NOTE: 126 gearboxes do NOT use EP oils, as the additives can cause erosion of some of the non-ferrous metals in the gearbox.