OK best of luck. I hope your problem will be resolved soon in sha Allah.
Yes it is gear oil. You should get it checked and change it completely. Always use Genuine toyota CVTi fluid.
"B" is for "breaking". You can use this gear while driving down the hill. It uses clutch plates to keep car in slow and in control. You can say it is a kind of auto-breaks or in simple words the car remains in lower gears.
"S" is for "Super". Putting car in "super" stops gear shifting. It increases pick. So if you want to keep car in the gear in which it is present, just put it in "super" it will stop shifting. You can use this to give full throttle like manual cars. For example start the car and push it into "Super" gear. Now give a quick pick to the car. When accelerated to a certain speed just shift car back to "D", it will change the gear to upper level. Again shift it back to "super", continue to accelerate and shift back to "Drive" and the car will move to next gear. So like this you can also use it for racing "ragra" kind of pick It will act like a kind of a manual shifting.
best of luck,