I am trying to pull four #8 stranded wires through a flexible PVC conduit that is 25 feet long. I can get the wires just a few feet into the conduit before they bind. I know there is a commercially available wire lube, but I do not have any. Can I safely substitute another product like dish soap or something? Or would that just be a bad idea and I should just get the real deal?
I'd agree with the comment that you may have another problem.
What size is the conduit?
Are you pulling or pushing (pulling is what works, pushing won't.) If you need something to pull with, use a shop-vac to get a rope through the conduit first. Braided hollow rope can be nice as you can expand the end and use it like a Chinese finger cuff to grab the ends of the wires (use tape as well.)
If the conduit is exterior and the wires are correctly rated for exterior conduit, you could use water as a lubricant. It's not a good idea to come up with a random soapy product, since the commercial products are tested for long-term compatibility with wire jacket material, some lube will remain in the conduit, and the wires are generally expected to be there for decades - over which time it's possible that the seemingly innocuous dish soap might have a deteriorating effect on the wire insulation, due to some component inconsequential in washing dishes (say, for example, the scent...)
The real deal is quite affordable if you can find it in quarts, which you generally can at most electric supply houses or the internet if you have difficult electric supply houses and your home improvement stores don't stock it.
The nominal diameter of #8 stranded copper with insulation is just over .21 inches. You are trying to pull 4 of these wires through 3/4 inch pipe. I'll bet the 4 wires together have a diameter of .5 inches (in the best case where the wires are still straight and have no bends.) That's a tight fit over 25 feet. I don't know if you are using a mechanical pulling device, but that would be very helpful if one is at hand.
It is pointless to try to push the wires that far. It would be like a shovel with a rope handle. As suggested before. Send a pulling rope down the conduit and attach it to the wires solidly. The trick suggested of using a rope like a Chinese finger puzzle is a good one. As for friction. A quart of the real pulling compound costs about 6 bucks and you will have plenty left for your next job. I agree that using dish soap is not a good idea. The insulting materials vary and the stuff will be in place for decades. Pinholes in the insulating material are a very real possibility and copper will start to corrode with soap. It's not likely there would be enough to degrade #8 wire, but why chance it.
Also, soap solution does not have as low a coefficient of friction as the pulling compound. After paying what you had to buy that amount of copper wire, spending the six bucks for a quart of the right stuff is chump change.