Don't take this wrong, I have two things to say that are for others out there also. First, if you live where you know it gets cold in the winter, and you have diesel powered equipment, why not plug them in the night before you are going to use that diesel powered piece of equipment? There are coolant heaters, oil pan heaters, battery heaters etc. on the market for exactly this purpose. Loggers have coolant quick connects on pickups and skidders and loaders so the pickup can circulaate warm antifreeze through the cold diesel eqwuipment for a while before they start it, why, because they can't afford new engines all the while.
Second, just supposing you didn't need that diesel powered piece of equipment that day, and you just started it because you wanted to, there is an old adadge that goes like this: If you don't need it, don't start it on a cold day. Nothing good comes of starting a stone cold diesel engine if you don't have to:
Now, if you aren't real PO'd at me, I'll give you my opinion on your oil pressure drop. Because I assume it was totally not plugged in to oil pan and coolant heaters, your motor oil was pure molasses, 15w-40 is terribly thick at below zero, some brands worse than others. If anyone doubts me, put a jug outdoors on a zero or lower night, it totally shocks me everytime I grab a gallon of it on a cold morning. It's just about as stiff as 30wt motor oil. That thick cold oil is being pulled through two pickup screens in your oil pan that lead to the oil pump and those screens may have a little crud on them, so they don't flow perfectly.
Your system is working really hard to get the oil flowing, it probably isn't getting a full flow of oil, cavitation occurs and pressure drops. You will probably be OK when the ambient temp gets up around 30 degrees.
The guys here that really know me will understand that I am not being a know it all, and I wish you all could come up and visit at my place sometime so we could share experiences, cause that is what this is all about, sharing and helping eachother out when we can. JT out