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Sounds like you’re well prepared... your gf is lucky to have you!!! I’ve been using surgilube based on my surgeons recommendation. I bought a 4 pack on Amazon and have gone through about a half a tube a week or so
Surgilube Surgical Lubricant Sterile Bacteriostatic Jelly - 4.25 Ounces Each (Value Pack of 4) https://smile.amazon.com/dp/B00JLIFY44/ref=cm_sw_r_cp_api_glt_fabc_ZQC55H358SESJE4BAVY1
I highly recommend Surgilube Lubricating Jelly Sterile. You are going to need a lot of it in the beginning for dilating. Amazon has it for $18.48 for a 4 pack of 4.25oz tubes here https://www.amazon.com/gp/product/B00JLIFY44/
I went through the 4 pack the first month. I had to dilate 4 times a day the first month.
You are going to need a lot of it. I highly recommend Surgilube Lubricating Jelly Sterile from Amazon
I have yet to experience a prostate orgasm either anally or through sounding. I've had a hard time getting anal play to activate much but I've also only used my fingers. I have some toys coming in that will let me try it a bit differently and hope to achieve a prostate O with those.
Via sounding, after getting use to sticking something in there (it takes a while), I finally got in far enough to feel something really unique. The first dozen sounding sessions were with a straight rod that was 6mm in diameter. I couldn't reach my prostate with the straight rod but I had a more snake-like rod that was 6mm and 8mm in diameter which could get around the final curve to start reaching the prostate.
The first time I used the snake was as uncomfortable as the 3rd time I used the straight rod. After easing it in enough I felt some minor discomfort near my perineum. That part of the urethra is less sensitive so I was taking an extremely cautious approach to pushing myself. The discomfort wasn't painful but I could tell that part wasn't use to the new diameter yet. All I did was gently, slowly pull out and push in for a while to ease that part of my body into handling the larger diameter. When I pushed in I noticed something faint. Besides the minor discomfort down there, I felt a slight need to pee (like all the pleasing sensations of needing to pee without the bloated discomfort) as well as some sort of tingling in my head (not penis, but cranium). It wasn't like a tingle in my muscles or anything. More like the first 1% of the buildup sensation of a normal orgasm. It did feel nice but was so faint and I was paranoid about the discomfort so I stopped.
Second time with the snake I felt a bit more confident and the discomfort was less. Again, I reached that point where something faint yet pleasing was building up. I didn't push too hard since I'm still new to prostate O and figured my body is still adjusting to the new size, but I was able to push a bit more this time. I don't know if I went deeper (it's hard to measure because there's still a couple of inches of snake sticking out when I get there but I plan on counting the balls to start keeping track). The sensation to pee was greater as well as the tingling in my head. It felt really, really good and I knew I was on the right track. If the first time was 1% of an orgasm, this time I think I got to about 3%.
The problem I'm bumping into is unfamiliarity with what will happen if I keep pushing. Most session with my sounding rods has been while I'm flaccid. It took a while before I built up enough confidence to try getting hard while the rod is in, and inserting the rod is such an invasive feeling that I couldn't keep it hard once I started trying to push the rod in. Also, the pressure in my urethra is greater when fully aroused, which means my urethra is tighter. That tightness is enough to make inserting a rod more painful. Too painful for me for now, despite being a masochist.
When I finally reached full mast while the straight rod was inside already, any clenching caused a bit of pain. The urethral walls are so sensitive. I did like the pain from clenching but I was barely clenching. I did some edging with the rod in but I was too worried about the clenching to try for an orgasm. As hard as I try, I usually can't NOT clench a lot when I reach orgasm, and I knew if I couldn't control it I could wind up clenching really hard and harming my urethral walls.
So, that's why I haven't tried pushing for a prostate O with the snake yet. It feels so good to gently poke at it directly like that, but I don't know what would happen if I reached prostate O while it's inside. I'm afraid any jerking or clenching would quickly make the experience really bad, so I'm giving my body an extremely long time to adjust before trying anything too serious.
I'll keep trying. It's a lot of fun and that pleasure I felt from reaching the prostate is absolutely wonderful. I highly recommend it. Best to read up plenty on it if you haven't already and start with something that is around 5mm (I started with 6mm). If you think your urethra is smaller, 4mm is good to start with. If you think it's a bit bigger, 6mm can work. I bought the snake first and realized right away that the 8mm balls were too much, so I immediately stopped and went with the 6mm straight rod which worked fine (still hurt the first few times and burning while peeing happened for a couple of days).
Edit: For lubricant to use with sounding rod (please read about sanitizing and caring for sounding rods! that's the most important part of owning a sounding rod), I use surgilube which you can get on Amazon for super cheap right here.
I had some interest and bought the $80 Rosebud kit (see here - no affiliation) and some bacteriostatic lube (like this). I was understandably nervous the first time I tried but found it surprisingly easy and enjoyable. I especially enjoy the feeling of going all the way in and resting at the bottom (on the bladder?) and the feeling of massaging the frenulum from the inside. I am able to cum with the sound inside with just a little external stimulation, and always pull it out to make sure I get a good flushing.
I've used the kit maybe a dozen times over the past 6 months or so since I bought it, alone and with a couple different partners, and the only disappointing part is I was only ever able to get the first two sizes in, and haven't see any stretching of the urethra. The third size up won't go in at all, so I seem to be stuck.
Besides that it's great fun. Letting the sound slide all the way in for a partner who has never seen sounding before is quite amusing xD
My urethra does feel a bit irritated after a sounding session, but it's back to normal by the next day. I highly recommend giving it a try. Just make sure everything is clean and well-lubed, go slowly and no sudden movements.