Here's what you should do:
1. Stop driving immediately: Driving with this condition risks catastrophic engine failure. The sludge indicates a breakdown in lubrication, and the fluctuating RPM/stalling confirms a problem with engine operation.
2. Have it towed to a qualified mechanic: This is not a DIY repair. A professional mechanic needs to diagnose the root cause of the sludge and the engine problems. The possibilities are serious and include:
* Crankcase ventilation problems (PCV system): A faulty PCV system can cause excessive blow-by, leading to sludge buildup.
* Failed head gasket: A blown head gasket can allow coolant to mix with oil, creating a milky or yellowish sludge. This is a very serious issue.
* Failing oil pump: An inefficient oil pump won't circulate oil properly, leading to sludge and potential engine damage.
* Contaminated oil: Using the wrong type of oil or an oil that has been contaminated can contribute to sludge.
* Internal engine damage: Worn engine parts can create debris that mixes with the oil, forming sludge.
3. Do NOT add oil or any other fluids: Adding oil without knowing the cause of the problem could worsen the situation.
4. Describe the symptoms clearly to the mechanic: Tell them about the yellow sludge, fluctuating RPM, and stalling. The more information you provide, the better they can diagnose the issue.
The cost of repair will depend on the cause of the problem. Ignoring it will only make the cost much higher in the long run. Get it to a mechanic as soon as possible.
