in my TJ i run the Castrol Syntec, it mite be expensive but ive got 75k on my jeep and no leaks or engine problems. i will not nor recomend to any of my costomers to run anything by Shell, Chevron, Penzoil or Quakerstate, the are a waxy based oil which can leave basically Sludge in ur engine unlike Valvoline, Castrol and Mobil 1 which is a mineral based oil, much better 4 ur engine
they are all technically "wax" based even though that is a far streach from what it actually means, dyno motor oils are from a chemical family called a parafin (it is in wax, but it isn't wax) basically parafin is kerosene, you can chain it up or out, up is a strait chain parafin, out is a bent or angle chain parafin,
A bent chain parafin exposes the orbital shell on the carbon atom where then chain 'bends' that creates a location that allows hydrogen bonding, the advantage to this is it sticks to metal and will protect a motor during startup, the disadvantage is it traps the tiny metal particles that float in your oil and create sludge, also these oils tend to be more viscous, so they need less Viscosity index improvement. if you think of an oil molecule as a bearing of sorts, this one would be L shaped, so it provides less lubricity.
This type of oil is also not compatible with certain oil detergents so it tends to not keep an engine as clean.
a straint chain parafin exposes very little, which makes it less viscous, but extremely slippery. less likely to stick to metal and doesn't create sludge, Valvoline and Mobile one are both strait chain pfs. so you know most strait chain oil add a certain amount of a bent chain as an additive to help with the startup. if you think of this molecule as a bearing it is as straint as they come which makes it slicker.
A synthetic oil is based on a strait chain pf, the difference is they get an extremely consistent base of oil, less then a 3% varient, as compared to a 15-20% variant in standard oils. They can carefully control the properties and the way an oil behaves because it's so consistent.
If you have a high mileage engine that has been using havoline/Penzoyl/Chevron/walmart/Quacker Stat/ you shouldn't switch to synthetic, it will F*** up your universe, it will cause the sludge to slide and it can plug things you don't want plugged.
If you use the others, you should switch before 50,000 miles, but not before 15000 (unless your new motor has roller everything then anytime you want) because pf oils cause rubber to expand the synthetic won't so once you switch you will find leaks you never knew you had.
I use synthetic on my Car and I did on my '02 chevy, it's good for 2-5 mpg and makes them start easier in the winter time. i've used Mobile and Valvoline synthetic and I can't say as I've seen a difference between the two.
