What exposure time do you normally use with your STi Mono and OAG-8300?

I recently switched from a Borg Mini 50 guide scope (250mm F5) and Orion SSAG to an SBIG STi mono and OAG-8300 on my wide field setup which is an Intes Micro MN-65 (165mm F5.5 Mak Newt). I knew going in that using an OAG makes it a more of a challenge to find a good guide star, but I went from never once ever having a problem finding a guide star in any part of the sky with a 2 second exposure with my old setup, to having problems in most parts of the sky with 3-4 second exposures, and getting star fade messages quite frequently. Thankfully my mount (MI-250) tracks pretty well and I have been able to get good images so far. So I'm curious what others find works well with their setups? How much of a challenge do you have finding a good guide star?

I set the pick of mirror pretty close to the suggested mark for an F5 system, and this did not introduce any shadow in my main camera when using a light box. I'm wondering if moving the mirror in would help with getting brighter guide stars? I've heard that flats will take care of the shadow as long as it isn't too drastic. The scope I'm using doesn't have a very large secondary mirror, it was designed for use with APS-c DSLR camera sensors in mind, so maybe illumination drop-off could be part of the problem I'm having?

I'm also using full calibration with 20 each of bias and darks for the STi.