I don't see a problem in draining it and re using it. Like Jim said just make sure everything is clean.
I'm a firm believer in thinner oil during the winter. It makes a night and day difference in how the engine starts and runs.
+100
There is a noticeable cold starting difference running 10W30 (I've never used 5W40 but predict it would be fine as well) in cold starts vs 15W40 conventional.
But I use 10W30 all year around anyway. When I got my truck, that was the recommended oil grade. Also, since my truck lives in a relatively cool climate now and no longer does long distance towing, it's an appropriate grade. I used API SF/CC 10W30 in mine from about '87 into the early '90s, and that included a LOT of heavy towing, then went to the then-new SF/CD 15W40 dual rated oil when I had trouble finding dual rated 10W30. As soon as it was readily available some years later, I went back.
By then I had learned some things about oil so made some tests. The first was to average engine oil temps over about a year. Oil temp seldom approached 200F but basically never exceeded the 212F point where the operating viscosity grade (the "30" in a 10W30) is measured. I also tested oil pressure at the same temp with fresh 10W30 and fresh 15W40 and there was only about a 4psi difference at 2000 rpm. Actual viscosity is measured in centistokes (cSt) but the "30" is a grade that fits within a range of actual viscosity. When oil is cooler, it is thicker and vice versa and if you apply normal operating temps to a viscosity chart, as I did, you can see approximately where the viscosity is running. In my case the average oil temps were between 170 and 190F and the 30 grade oil I was using (Rotella-T Triple protection, which is thick in grade, I now use the 10W30 Rotella T-5 which is similarly thick in grade) was most often running in a 40 grade territory anyway. Using a 15W40 viscosity chart with those numbers, it would be running in 50 grade territory most of the time. I had an oil temp gauge hooked up for about two years and my logs show over 200F temps only a few times during that period and never over 212F. If the oil temps exceed 212F by too much then the oil gets
too thin and that can be dangeroo long term.
There are a couple of other standards to apply as well, one of which is fuel dilution. Enough of that an the 15W40 gets dangerously thin and 10W30 gets dangerously thin a bit faster. I've done enough oil analysis over the years to know that this is
not a problem on my truck and I do a sniff test on the dipstick regularly so it won't catch me by surprise if something goes wrong suddenly.
Another factor is shearing, whereby the Viscosity Index Improvers in the oil are mechanically "sheared" or worn out. These VII are what gives oil it's multi-grade/all season performance. Eventually these VII polymers wear out (in any oil) and the oil begins to revert back to it's base grade, which is on the thing side of the equation. The wider the difference between the "W" number (the "10") and the summer (the "30"), the more VIIs are in the oil. A 15W40 actually has more of a gap than 10W30, so in theory would shear more. In practice, shearing is not much of a problem these days because of the improvements in the VII. In old school diesel like ours, the oil usually thickens first due to soot. We used to see a fair bit of shearing in the old 15W40 oils but it's not common these days, even in the conventional realm.
