I have a mantis and I changed out the stock diff fluids from when I got it to 10k center 7k in front and 3k in rear. It seemed like it did not make a difference the first couple of times I drove it but now it wants to fishtale alot and spin out in the turns and I mean pretty bad. Also when I come out of a turn and into the straight when I am able to hit the throttle all the way it swaps side to side pretty good like it wants to come around but does not. I have checked the front diff already and everything is fine and it is not in 2wd, its not that bad. I still am not too familiar with what the different fluids do. I think I should change the rear diff fluid to a thicker but how much? If I change it to 5k will that make a big difference since I had 3k in it already? I am thinking of putting 10k in it. Or should I change the center diff fluid which I think really only tunes the front wheels. Please help!

just change thr front to 5k and you should be fine.

What is your reason for that? I figured since the back was spinning up to thicken the rear diff fluid.

right now your front is too thick. with 10k in the center and 7k in the front the center can't spin the front as easy as the back. so your getting a little too much power to the rear. most of the time you should run 3-5k up front. a good starting point is 3f-10c-5r. you want some powerup front and a little more in the rear. right now you have it backwards. understand what i'm saying?

So whether I lower the front by 2k or raise the rear by 2k, 2000 difference in weight will make a big difference?

Everyone by me always runs thicker oil in the front then in the back that is why I have it like that and it is like that on the other truggies I have but this mantis is the only one giving me a problem. You are saying opposite.

According to this I found on this site I should thicken the diff oil in the rear:

There is actually lots on here about diff oil s if you search for diff oil s, but for those who may have missed the info here is a quick summary

Front
Thinner :
increases steering into corners (off-power)
if oil is too thin the steering may become inconsistent, especially it can lose forward traction (and steering) during acceleration out of corners

Thicker:
increases stability into corners during braking
increases steering on-power at corner exit

Center
Thinner:
front wheels unload more during acceleration
decreases on-power steering (reduces oversteer)
easier to drive on rough tracks
if a high-power engine is used you could waste too much power and sometime “cook” the oil in the center diff erential because it “overloads”

Thicker:
more all-wheel drive effect
better acceleration
increases on-power steering (reduces understeer)
better suited on high-bite, smooth tracks
car can be more nervous to drive especially if a high power engine is used - you might need to be smooth on the throttle

Rear
Thinner:
increases cornering traction
increases steering into corner

Thicker:
decreases rear traction while cornering
reduces wheelspin

yes 2k will make a difference. but if you have everything too thick it won't turn for anything. lower the front to 3k and you will see a very big improvement. 10k in the center is pretty much the norm so your ok there.5k in the rear is ok but 7k in the front is way too thick. check around on different forums and see what the pro's are running.

that has some good info but setting up your diff's can be tricky. it depends on your driving style,tires,track conditions,engine your running ect ect. i found that a good starting point is 3-10-5. see what the guy's at your local track are running that will also help.

Actually I finally took my diff apart today and it was EMPTY! no diff oil at all.. Funny thing is that the case that enclosed the complete diff on my mantis is not wet like it was leaking or anything. Only thing I can think of is it had a slow leak and after me running it everytime I would just wash it off and maybe never noticed that it was wet around it and it eventually leaked all out. That it most likely my problem why it was spinning out easy. So I put new orings in it and filled up with 5k oil and we will see when I get it installed.