I have the losi scte truck and was wondering if switching from a plastic spur gear to a metal one will make any difference? Is this recommended? Also im currently running a 15/40 setup and see they also have 48t spur gear hiw will the 48t differ from the 40t? Will the motor temps rise or drop as opposed to the stock 40t?

Metal spur should be more durable, but also a bit more noisy than metal pinion/plastic spur combination.

Usually increasing the teeth count on the spur gear will reduce motor load, and temps.

So increasing spur gear tooth count will help lower temperature but how will it affect speed using same pinion gear?

If you increase the spur gear teeth count but keep the pinion the same, the wheels will turn at lower rpm, so you will lose top speed, but gain on acceleration. Same applies when you lower the pinion gear teeth count and keep the spur the same.

Metal gears are stronger, but will wear more over time since it is metal on metal contact.

Would lubrication help to prevent the faster wear? Assuming spur and pinion are shielded from dust etc, as grease and sand are no friends....

Possibly, but it'll likely just fly off with the speeds the gears are turning.

A good grease should leave a small film on the gears, after the excess flies off. Recently I put some grease on the spur and pinion on my Lazer ZX Sport, and it made the transmission more quiet, and the car slightly faster. I had the same good experience 25 years ago, with the Optima Mid, never had to replace a spur on that car so far. But that is metal pinion on plastic spur, so different combination, but the gears spin at the same RPM.