What do you do with the oil in your oil cooler, oil lines, thermostat housing when you drain the oil in your engine and you have a thermostat bypass on your oil cooler? Do you disconnect a line from the cooler and drain it or leave the old oil in the cooler?

I depends on how your thermostat is set up and the routing of the lines.

I just drain the pan and change the filter. You will get a large majority of the oil out. There is probably a quart or so left in my lines and cooler.

Some thermostats only close off the cooler 95% or so, therefore a small amount of oil circulates at all times. If everything is routed properly theoretically everything could drain back into the pan after you remove the filter.

One thing to think about - if you empty all of the lines, coolers and filter the engine will be running for quite some time with no oil pressure when you start everything back up and the oil pump has to fill the whole system.

If you do choose to drain everything, fill the lines, cooler and filter with oil before you bolt everything back up to minimize the time at zero pressure. Some of the engines with horizontal filter positions obviously cannot be filled completely.

Or you could buy a pre-oiler system. It will drain the oil in a minute and fill the system back up before you turn the key!

Steve