Strange fuel gauge reading on 560SEL

[Snibble]

The last fill up, I filled entirely and added a bottle of Techron Chevron to clean out the system after a long hibernation period. Today, as I'm nearing the reserve mark, I noticed the gauge now dips below the R but does not light up that its at reserve. Then it goes up a little and then it fluctuates from slightly above the R and then below the R... while the reserve light has not lit up. I'm planning a fill up today, but I'm worried that my sender could be dieing for some reason.

Car is a 1990 560SEL with 68xxx miles.