The Mazda recommended transmission fluid weight is 75W90.  Ensure the transmission fluid you pick explicitly states:  "safe for use in modern synchromesh transmissions".  Not all gear oils are safe for bronze synchronizers in the transmissions.  Sulfates are typically added to gear oils for higher load capacities.  Sulfates gives off a "rotten egg" smell, so if the fluid has this type of smell, don't use it in your transmission.  Sulfates attack bronze metals, so bronze synchros are eventually eaten.  Typical experiences are increased noises and crunching while shifting into gear.  If you are experiencing increased noises and resistance shifting into gear, and you have just replaced the transmission fluid, it might be a good idea to change to another brand.  It is highly recommended to use a synthetic transmission fluid for your FC3S.
FC3S Pro:  Mods - Transmission Fluid

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