I was going to ask what the temerature was like where you lived. DO you have a block heater? If not you can also use a trouble light. A 100 watt light bulb will usually warm an engine almost as well as a block heater ina few hours. Just turn on the light and stick it near the oil pan, making sure it isn't touching anything easily meltable. Like Larry said, these might be extreme measures, but it sounds like you've already put a lot of work into this rebuild, a little more during the break in sure couldn't hurt.

Let us know how it goes.

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired