Mikuni BST40 slide guide
Notice the ~0.020" (0.51 mm) deep depressions in the bottom corners/concentric to the bore (circled in green), as well as the four circular ~0.023" (.58 mm) depressions.
Contrast this with the same areas of a slide guide from a KTM 640LC4 with under 20K miles (32,187 km) (claimed)(note: wear was accelerated by slide lift hole drilling). The depressions in the bottom corners/concentric to the bore (normally found in the area bounded by the red circles) as well as the four circular depressions are worn completely away. The plastic is worn completely through adjacent to the bore at 10 and 2 o'clock (when viewed in the installed position). Maximum wear measures 0.085" (2.16 mm), which works out to .0042"/1K miles (.07mm/1K km) (!).
Emulsion tube wear sets in when the circled depressions are somewhere around .010" (.25 mm) or less deep, and causes overly rich mixtures in the lower portion of the rpm range. Measure with the tail of a caliper. Zero as shown, then find and measure the lowest spot in the immediate surrounding area. Repeat for the other side.