Oil drain

**Tiny Tim** · 20th July 2011, 06:54 PM

To our learned brethren
I've often wondered why manuals always tell you to warm up the engine/gearbox before draining the oil. Surely the best time to drain would be after the hot engine has been resting, say overnight.

Tim

**JDNSW** · 20th July 2011, 07:58 PM

There are two very good reasons.

1. When the oil is hot, it has lower viscosity, and so more will drain out more quickly.

2. Probably more important; foreign material held in suspension drains out with the oil when it is drained just after running - if left overnight, this will settle out onto the bottom of the sump or gearbox and will be left behind to contaminate the new oil.

John

**isuzurover** · 20th July 2011, 08:32 PM

Originally Posted by JDNSW

There are two very good reasons.

1. When the oil is hot, it has lower viscosity, and so more will drain out more quickly.

2. Probably more important; foreign material held in suspension drains out with the oil when it is drained just after running - if left overnight, this will settle out onto the bottom of the sump or gearbox and will be left behind to contaminate the new oil.

John

What JD said.

**rick130** · 20th July 2011, 09:30 PM

Originally Posted by JDNSW

There are two very good reasons.

1. When the oil is hot, it has lower viscosity, and so more will drain out more quickly.

2. Probably more important; foreign material held in suspension drains out with the oil when it is drained just after running - if left overnight, this will settle out onto the bottom of the sump or gearbox and will be left behind to contaminate the new oil.

John

Originally Posted by isuzurover

What JD said.

Ditto.

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