One of the things I have noticed on drain/fill plugs on bikes is that sometimes they come all copper washers, sometimes they come all hollow tube style crush washers and sometimes you get a mix with no real rhyme or reason for it. Either style, in any material, plastic, (yes, they really do make plastic crush washers!), aluminum or copper will do the job of sealing and protecting the threads.
The idea behind crush washers of either style is not really to seal the hole, but rather to save the threads from potential damage. I hear what you're saying
@Wheelin about using the torque wrench. However, there is a real potential for over tightening with a torque wrench on these particular applications. Torque specs are for dry threads. But the threads on the drain and fill plugs are lubricated by the very oil going into and out of them. A dry spec torque is much too high for a lubricated thread. Ballpark is 20% less for lubricated threads. I say ballpark because there are too many variables involving size, lubricity of the fluid and contact materials to really give an exact value.
Add to that, most torque wrenches, even very high quality ones, are really only accurate at one point in the range. The standard procedure is to zero the torque wrench accuracy at the middle of the range. Typically, the high end is closer than the low end, where the biggest error is usually found. I've calibrated a lot of torque wrenches when I worked as a machinist in industrial manufacturing. Many riders are using ft/lb torque wrenches for these low torque value fasteners because most of us don't need multiple torque wrenches and just have one for everything. This typically means you are at the bottom of your wrench's range, or sometimes below it, where it's really not that accurate.
Keep in mind that torque values are listed for two primary reasons. Very few applications really need an exact torque. Bearing caps are one example of something that needs an exact torque where over and under can cause serious issues and failure.
Most torque values are for the two primary reasons: So you don't break it off, and so it doesn't fall off. For the drain/fill plugs it's so you don't break it off/strip the threads.
So here is an alternate method to consider in lieu of a torque wrench. Thread the plug back in until it's snug. Now put your wrench on it and tighten 1/8 turn more. You should fell it solidly bottom against the case and then require more effort as you tighten that last small amount. This is enough to ensure it is fully seated against the case and that additional 1/8 turn will be compressing the washer. This method is NOT for a new hollow tube style crush washer, only for flat style or re-used hollow style crush washers.
In wrenching on other people's stuff for money, I would see drain plug washers in various states of use. It was not uncommon on cars that were typically taken to the 'quick lube' type shops to find drain plug crush washers paper thin from multiple over tightenings. Those crush washers saved the oil pan threads during repeated & sometimes excessive over tightening, but end of life happens. If your crush washer is not significantly deformed when you remove the drain/fill plug, it's fine to use again. If you're the one doing the wrenching and you can visibly see that your copper or aluminum crush washers are deformed, you are over tightening the drain/fill plug, regardless of if you were using a torque wrench correctly set to the book value or not.
Sorry for the long missive. That's just my opinion and experience. I wrenched for money for about 12 years. I worked as a production machinist in +/- .0002 tolerance work for 20 years in hydraulic tools. I've also worked as a gunsmith and at a knife manufacturer over the years as well as a handful of job shops seeing all kinds of one off stuff.
