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I recently took my horn in for a chemical cleaning. The tech commented that the slides were covered in oil, but did not seem to have much grease on them. I have noticed over the past year or so that the grease seems to break down much quicker than in the past. I'm thinking that over time the slides may have worn down so that they don't fit quite as tight, thereby allowing oil to get in. Any thoughts on what may be going on or how to deal with this issue.

Very easy: So What ?!? Embrace it.
Slides don´t need grease, they need a lubrication that will keep them mobile and provide a seal to prevent air from leaking out.
Back in pre-industrial times (and for quite a while into mass production) when tolerances were wide, grease (deer serbium) was the best and readily available product to meet the requirement.
With modern perfection in diameter tolerance as well as surface quality, an oil with reasonable viscosity (rotary oil) can perfectly seal and lubricate, and facilitate slide-pulling during performance in the process.

It´s the same with bottles of wine as well as whisky:
Those don´t need a good cork. They need a good stopper, and cork used to be the best widely available variant in ancient times. Those suck, though, due to capillaries and pores that won´t provide an airtight sealing and are prone to insect attacks. Forget that "but Tubeast, wine needs to breathe"-s**t. That was a sales excuse, and the wineries knew it.
Which is why they dipped the expensive bottles in sealing wax. Had plastic stoppers or glass stoppers with rubber seals been available back in the day, they would have screwed those corks (pun intended).

I've posted all to redundantly that I try to avoid using grease on my slides, I try to use valve oil on all of them, and occasionally I might reluctantly add a very little bit of 30W oil to one or two of the slides.

Well my new tuba arrived with such thick grease on the slides that I literally have trouble, strength-wise, moving them, just to get them out. One of these days I have to clean them off (but there is also grease on the tubes they are in) and get something a bit more slippery on them. They have a very tight fit, being a new Miraphone.

Just keep re-greasing as necessary

To answer your question, if you need to dump a lot of condensation and/or oil your valves every time your going to play, then the grease won't last as long. In my experience. Hetmans number 8 or the 7.5 in syringe form are quite nice though.