which oil would you recomend for my 2.5 wrangler? have just changed the oil and used castrol gtx magna 15/30(non synthetic/recomended for cars with more than 65000miles,) also used a filter from motor factors not mopar part....(just ordered 3 k+n filters from quadrotec as i change oil every 6 months even though i do very little milage... oil creaming up very easy.... what ever the oil was in there before this didn't happen......should i change to a semi synthetic or fully synthetic or 20/50 that i have seen for classic cars........