Type of oil

i have a 98 corolla VE with 236,000 miles on it, the car runs great and doesnt show its miles, im the second owner and the guy that owned it before me said they were mostly all highway miles, and ive put 20,000 miles on it in 8 months. Currently i run pennzoil 5w-30 and/or 10w 30, depending on what i have when i change the oil. i was wondering if it would be worth it to upgrade to a full synthetic such as mobil 1 high mileage or pennzoil premium full synthetic etc, in order to keep the engine running longer and clean off some sludge that must be in the engine, or should i just stick to conventional oil and upgrade to pennzoil high mileage or something similar since it has a little bit better protection and some detergent in it. i would think mobil 1 high mileage would be best, but i dont know if its a good oil for the corollas engine? or if synthetics are worth it at all. people tell me it is worth it and their engines ran better when using it, but im not sure. any advice or input would be appreciated, thanks.