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So I was replacing my spark plugs today, and noticed that when I went to replace 3 of them, the holes were full of oil. So I'm guessing either the previous owner dumped oil down the holes, or more than likely, its my valve cover gaskets. In reading the manual and some posts on here, it looks like I could do the front one myself, but the rear one looks like it would be a little much for me to do.

So here is my question:
Does it really matter if I replace them? Can I just leave them to leak? What is the worst that could happen by not replacing them?

I got a quote from a few mechanics, including a mazda dealership, and they all seem to be about the same cost, about $500! So that's why I'm questioning if its really worth it or not.

Thanks in advance for the help

You can get the gaskets for around $70, and spend a few hours doing them in the driveway. It's really not all that hard. The rear isn't harder, just more time consuming, because you also need to remove the IM.

So, when you get the gaskets, make sure to replace the IM one as well

Yeah, you need to replace them, and it is easy There's an online manual. I suggest getting pictures of stuff as you take it off, so you get it back on the right way.
If you don't, your car will run badly, get bad mileage, and of course, lose oil. Don't let someone charge you for doing it! Look at it as a fun puzzle.

a cheap fix is to take the covers off, clean them really well where the gasket goes. clean the gasket with some orange pumis soap from crappy tire, put a thin layer of ultra grey gasket sealant between the gasket and valve cover. clean the heads off where the valve cover gasket sits and put a thin layer of gasket silicone on there and put back on, torque the covers and you're done. if you continue to let it leak you will wreck your plugs and wires, plus it wont run as well since it isnt getting spark very well going threw the oil.

I am in agreement with replacing the gaskets. You can do it yourself its not that hard. Just a few more bolts and a couple more gaskets and an extra couple of hours. Taking digital pics as you go is a great idea. Not a bad idea to bag & label your bolts as well. Taping & numbering vacuum lines prior to taking pics will make it easy to put them back where they go. I'd recommend replacing your PCV valve at the same time. Sometimes if they arent working right.. you can have a bit more crankcase pressure and that can force oil past gaskets and even past the PCV itself. If you find oil in the intake.. I'd recommend cleaning it up as well. Brake cleaner and carb cleaner cut through that stuff pretty good. Just dont get any in the sensors.

Start with the front gasket.. make sure you clean everything extremely well. The cleaner your mating surfaces.. the better the seal. Soft wire brushes will help a lot with this. As an additional bonus.. the feeling of accomplishment you will get when the job is done will be one you wont forget. I mention it cause whenever I tackle a new job I have never done before.. and complete it successfully.. it makes doing the job sooooo much more worth doing. Plus.. then you can say YOU did it.. rather than having a shop do it

Tunes67

And if needed, please contact me through PM with questions along the way. I've had to do this twice over the past year (two different engines, my K8 then my DE). I dont mind helping at all. Bagging and labeling is a huge help, especially when removing the IM. Maybe consider cleaning/painting/powdercoating/polishing the IM while it's off... Just a thought. It is a great idea doing it yourself since the job price will more than quadruple with the labor and parts.

My worklog has some pics of the job as well if you wanna get a glimpse of what to expect.

agreed, but there are sooo many people in Canada that should be close to you- offer some beer and a meal and get a hand showing you how to get it done always more fun with that way.

Don't forget to silicone every point where the gasket changes direction. Use O2 sensor safe silicone. Bought mine at Walmart.

as for gasket maker- get a shop to sell you a tube of Honda Bond- stuff would seal a torpedo hole in a tanker ship underwater in the artic. Honda shops use it and you can prolly talk a tech or the parts guy into selling you a half used tube on the cheap. Full tubes are $ and they dry up pretty fast after being opened (like a couple weeks).