I changed the oil on my 100,000 mile '99 SL2 last week, topping off my last 2 quarts of Pennzoil natural w 2 quarts of NAPA, w turned out to be a synthetic made by Valvoline. When I got back from my 2nd 100mile high-speed road trip since the oil-change the car was huffing & puffing & blowing steam like on old-fashioned IronHorse, so I'm wondering if synthetic & natural don't mix(the car was running perfectly b4(except for the transmission)). Perhaps the synthetic loosened up alot of sludge w is now clogging the filter causing overheating by diminishing oil-flow? Can I change the filter w/o changing the oil (i.e.how much will run out when I remove the filter)?

TIA