Long variable names

Mathematicians normally stick to single-letter variable names. But when they don't, it can be hard to tell if there's a missing multiplication symbol or not.

Some people use an upright font for long variable names to distinguish them from single-letter variable names, drawn in italics (see Use of different typefaces to convey meaning)

Examples
\[ speed = distance \div time \]

\[ \text{speed} = \text{distance} \div \text{time} \]

Is it OK to mix single-letter variable names and longer ones?

\[ x \text{speed} \]