Here at Specialized Dynamics, we use synthetic motor oil as a lubricant for all of the custom rifles we build. We’ve been using it for 15 years.
Let me explain, synthetic oil can assist in providing protection to equipment in high temperatures. It’s because synthetic oil has a high viscosity index, which allows the viscosity to be mostly consistent. Synthetic motor oil is a lubricant, there’s a reason why there are cars and trucks on the road with over 300,000 miles on them and still running good. With that said, its only a lubricant, not a rust inhibitor, not a cleaner, just a lubricant.
There are a lot of products out there claiming to do it all. CLP: Cleans, lubricates and protects. I have said it many times and tested this many times, there isn’t one product that does all these well. Synthetic motor oil only lubricates and creates a barrier between the bolt and the upper.
The guys up north, who hunt all winter in the freezing temps, negative and below zero temps, it is not recommended to use any type of grease or a thicker lubricity oil, as it might freeze and keep your bolt from cycling. Summer time, too weak of lubricity will cause excessive ware on your internal parts.
When lubricating your AR15/AR10 bolt carrier, look at the rails on either side of the carrier, just put a small amount on those four rails(2 per side) and thats the only place you should lubricate your bolt carrier group. Don’t put any on the gas rings and definitely none on the bolt face or lugs.
For the other parts of the CLP, we use other products in the shop. Synthetic motor oil should only be used to lubricate the internals of the firearm. We use KG Industries carbon remover and bore cleaner(copper remover).
Try some different products out for yourself and if you have one that you already like, keep using it.
The Specialized Dynamics crew all use synthetic motor oil as a lubricant for our guns, give it a try, worst case scenario, if you find your bolt to be dry out in the field, take some off your dip stick and try it out.