The 5VZFE on my '98 looked similar to the OP at about 105k miles. I purchased the vehicle at 95k and changed the oil every 5k with dino oil. The previous owner seemed to have done the same with what invoices were handed over to me. At 105k a tiny (1mm?) oil passage in one of the cams clogged and the forward bearing dried up, burned itself and the rubber seal up, and blew all of the engine oil out in my driveway. The journal isn't replaceable so a new cylinder head in addition to the cam would have been required. For a little more than the price of replacing a cylinder head and a little less than replacing both I got a rebuilt motor put in. I'be be really careful doing anything too aggressive trying to remove sludge. It only takes one little flake to kill your motor. Im not sure why the valve train and valve covers sludge up like they do but running synthetic for the life of the engine and <=5k OCIs seems like cheap insurance.