VAC goes to ~11 microns...then stops

We have an Optima TLX here in our lab...manufactured 2011, purchased in refurb/used condition circa 2015. We don't have a service contract so we are kind of on our own for maintenance (which is where I come in, as the only EE in a biotech lab). Anyhow our ultra recently started putting up VAC error that could not be cleared. I used the 411 code to display vacuum value and it was only getting down to about 2000 micron. 

I checked the vacuum pump oil level display and it was very low, oil was black w/ lots of sediment. Was able to safely remove pump, drained old oil, added new oil (of the correct type, from Ideal, per the pump labeling) to flush pump, then drained that after running until hot and replaced w/ clean oil again. After replacing pump in unit with fresh oil, unit achieved ~11 microns after leaving powered on for several hours, but seemed to be stuck there refusing to go below 10. 

Today I am planning to double check and hopefully re-seat the vacuum hose connection with some high vacuum grease. It was pretty difficult to get that hose off in the first place, I ended up having to cut the end and lost about 3/4" in the process (though there seems to be enough left for it to go straight up from the fitting at both ends). 

But - I am wondering what the likelihood is that the diffusion pump also needs service/an oil change? How often is that generally done? And does the apparent 11 micron vacuum value suggest anything in particular re the diffusion pump?

Another thing: I do see some small surface(?) cracks in the larger rubber hose leading from the diffusion pump to the chamber. They don't seem to penetrate all the way through...but seeing them is worrisome, so does anyone here have a good online source to order these replacement hoses? Thanks in advance...

- Corwin