Found on Reddit, should be a good starting point.

First and foremost: Radiator replacement and trans/coolant flush to prevent smod. I’m sure we all know what SMOD is but in case you don’t, 05-10 X’s have Calsonic radiators with a chance of having faulty trans-coolers. When the trans cooler leaks coolant gets into the trans fluid and eats away at the friction material in the clutches, eventually shorting your TCM and ruining the transmission.

second: Timing chain guides (also do tensioners and chain, water pump while in there) 2nd gen X’s start to get a whine because the timing chain guides are made of plastic and wear out, then your chain starts riding on metal. If its starting to sound like you have a supercharger under the hood you’re definitely at the point of changing.

Other:

Change all fluids. By 150k you should have changed the transmission fluid, coolant, power steering, gear oil in the diffs and trans fluid in xfer case, brake fluid, etc. From my experience the only trans fluid I could find that wasn’t Nissan brand that met the requirements was Valvoline Maxlife ATF. I’m sure there are others but I couldn’t find anything else that met nissan matic spec besides the actual Nissan fluid and the valvoline Maxlife ATF. Obviously it goes without saying you need to change your oil every 3k miles (5k if using high quality full synthetic).

by 150k you should have done a basic tune-up (spark plugs, coils, and wires). I’d also recommend a fuel system service at this time, A fuel system service is where you add a can of seafoam (or equivalent like gumout etc) to the tank, and run another through your brake booster vacuum line ( or can spray in the intake tube, either works but brake booster hits the injectors more directly) while idling at 1500-2k rpms. Drip the can in slowly with an IV drip and watch the idle. After the can is gone, let the engine hot soak and then crank it and give it a few good revs. this helps blow all the carbon out of your fuel system, and clean your injectors to get them spraying nice an evenly. It’s recommended to take the vehicle on the highway for 10-15 minutes after this to help clean even more after the service. Doing this can really rejuvenate your MPG and get it back a lot closer to where it was when the vehicle was new.

Cam/crank sensors tend to go out in Xs and can leave you stranded. It would be wise to replace them if you haven’t done so.

Rear diff breather reroute (if you offroad, or drive in water) Reroutes the diff breather to a higher spot and puts a small filter on it so you wont suck in water to your diff if you do a river crossing or go through some deep puddles.

Evap cannister clean/replace/reroute. the Evap cannisters are known to clog and cause an emissions code (I think p0443?) and can also make it hard to pump gas, your pump will click even though your tank isn’t full. It can be good to replace the cannister, or remove and backflush with air. Where they are located they vent to the frame and lots of dirt/debris/water can get in there. A lot of people reroute the vent behind their tail lights

U-joints. By 170k my U joints were toast, I mean completely disintegrated. If you hear a click when you put it in to reverse, or a wobble when accelerating around 40-60 mph (keep in mind you can also get a wobble from other things like unbalanced tires, etc) your U-joints on the driveshaft could be going bad. Get the ones with grease fittings so you can service them and they’ll last the rest of the life of your vehicle.

Brakes: we all know about brakes.

Ball joints: this one is a bit more work, but if your ball joints are all dried and cracked you can get some new ones with Zerc fittings so you can actually grease them and they’ll last a lot longer.

Gaskets/Seals: there are a ton you could replace but from my experience the valve cover gaskets like to leak, so do the rear main seals and the seal on the oil filter housing (or is it the oil cooler? I can’t remember, either way its the spot behind your oil filter).

Suspension: if you’re at 150k you might be starting to get a little sag from your leaf springs stretching and possibly your shocks starting to wear. You can do an add a leaf (I got the all dogs offroad kit) or a full leaf spring pack. Lots of people put bilsteins on as a fairly affordable/reliable suspension. Nisstec has great suspension parts for Xs also.

Bushings: around this age a lot of the bushings start to go bad. You can look at your sqay bar bushings to get a good indication. If they look extremely dried and cracked then your others may too. There are tons of bushings and rubber parts you can replace but a few i can list are: sway bars, trans/diff bushings, bump stops (can also opt for different size bump stops). I would also recommend replacing the two rubber stops under the hood. If your hood rattles when driving down the highway it’s because those stops have worn down. It’s super annoying but new ones are cheap and easy to put on.

Air filters: your car has an engine and also a cabin air filter located on the passenger side beneath the glove box. 9/10 they never get changed and look disgusting. That’s what you’re breathing when you turn on your air or heat so I highly recommend changing that.

Cup holders: this is an offbeat one but If you have the GIANT cup holders and wonder why nothing fits it’s because X’s have inserts that I guess most people lost. you can get them on amazon for a few bucks and are a huge quality of life improvement.

Air struts for the hatch: I bet by 150k your hatch doesnt just pop open anymore. Replace your air struts on the hatch for a huge quality of life improvement, bonus points if you put struts on the hood so you don’t need the hood prop anymore.

I’m SURE I missed a ton, but I think this should give you a pretty good starting point. Let me know if you have any questions or need any recommendations, It was a lot to type so I didn’t want to get into a ton of specific parts and brands.

EDIT: You didn’t specify which gen you had but all of this info is for 2nd Gen X’s.