Well, 10/30 isn't really thicker than 5/30. Well, it is and it isn't. the first number indicates the weight of the oil when cold, the second number indicated the weight when warm. So when you first start the car, the 10/30 oil is thicker, but as soon as the car warms up, they are exactly the same. The colder the oil gets, the thicker it becomes. 5/ oil lends itself to easier starting, but in the summertime this isn't a big deal. What I tend to do is to use 5/30 in the winter, since it is cold, I need something thinner so then engine starts easier, and I use 10/30 in the summer, since it is warm and the 10/30 provides some more protection until the engine gets up to operating temp.
See the Oil FAQ in the misc./articles section if you want some more oil related info
Just have to correct you on one thing Moon, the numbers dont tell you the weight of the oil, but its viscosity, or ability to flow.
Also, If you do put Synthetic oil in your Cav for the first time, make sure you use some cheap oil and engine cleaner, so when u change the oil, u can pour this in and let it run for a while, to clean out all the dirt and previous oil and then take it out befor you put the Synthetic oil in there.
ALso, if you do put synthetic oil in your car, make sure that you use a high-performance oil filter too, so that If you leave the synthetic oil in there longer than the regular oil, you have a filter who can still handle the dirt.
I got most of this straight out of my Auto-tech book, so this should be right "if" I remeber correctly.
[ This Message was edited by: johnyg on 2001-10-24 20:48 ]
I would be careful switching over to synthetic on a high mileage engine. It is possible that you have some leaky seals that are are basically being clogged by the dinosaur oil so that they aren't really leaking, but switching to synthetic will cause the leaks to break through.
Sorry about using weight in place of viscosity. That is what I meant