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What Causes Moisture on the Oil Fill Cap?
 
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Question:
What Causes Moisture on the Oil Fill Cap?
Moisture under the oil filler cap; there are drops but it’s not creamy looking. This is from a 5.7 Dodge Ram 4x4 and my neighbor has the same engine in his truck with the same problem. Another friend mentioned that synthetic oils absorb moisture. What causes it? Thanks
-- Fred Birsner, Greenbackville, Va.
Answer:
Without more detailed knowledge of your situation and vehicle operation, the most likely cause for this condensation is air humidity, which then condenses under the fill cap.  Water may be making its way into the engine through the air intake and is condensing at a point where it is cool enough to fall out.  As for absorbing water, all API Service Category motor oils are formulated to be able to tolerate and hold water in the oil if needed, rather than let the water sit undisturbed on metal surfaces and potentially form rust.  It is not the intent of any motor oil (conventional or full synthetic) to actively absorb water.
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