I would try a slightly thicker oil as you said first, as long as you're not feeling any power loss from the age and it's not a track car I wouldn't bother just use the slightly thicker oil and check it like you probably are doing, toyota suggests on new cars to check the oil at least every time the car is fueled which is probably a bit much I think I don't even check my landcruiser but I know it won't burn through 10 liters of oil in the 2500kms I change the oil in it, haha.