I have a 99 Mazda Protégé that I bought when it had 128,000 miles on it. Since then, I have changed the oil every 5k with Valvoline conventional 10w30. I would like to use Synpower in it since I already use that in another vehicle and it would save from stocking 2 different oils in my garage. I also would like the cold weather protection this winter. What I don't want is a leak. I am not sure of the maintaince history prior to my buying it with 128,000 miles on it. There is a bit varnish in the valve cover, but nothing out of the ordinary for the 196,000 miles it has on it now. What would you do? Stick to the conventional or go with Synpower?