I'm fighting scale on my Meyer's Lemon and read somewhere that you shouldn't use horticultural oil on inside plants that are not exposed to rain. The post said that the oil coats the leaves and keeps them from breathing if it is not washed off. Is this true?

I'm just not finding all the scale, trying to wipe them off with alcohol by hand. The plant is in my office at work and I can't shower it off or take it outside and I want to kill this scale before winter!

Not true. I, and many other people have used Horticultural Oil (Ultra Fine or Volk Oil) for many years with no problem. I have never washed the oil off with water afterwards. The spray mixture, at least with Ultra Fine is 1 per cent oil and 99 percent water. The only precaution you need to observe is to spray EARLY in the morning, or late in the afternoon, but never in the heat of the day. Late in the afternoon is better. Although I don't recommend it, people frequently use Horticultural Oils as a leaf shine. - Millet

There are evidences that oil sprays indoors can lead to leaf drop, because of plugging the lef pores of the leaf...
So the precaution is to wash the oil by using a water-soap-spiritus solution off....

But I also think, it's maybe because of differnece in oil here in europe to oil in US....
I made the experience, that oil willl not get off the leaf surface indoors, being a very nice sticky surface to dust, making the leaves more unattractive and thick sticked in dust and dirt, as using oil and rinsing the oil with soap off, which washes also dust and dirt away....

But expericnes can and will differ...