Originally posted by jeremy
if you are changing your oil at 3k then there isn't a large enough benefit to justify the extra cost.

Originally posted by 1987RX7guy
you can run sunthetic but it will build up carbon and crap since synth burns really bad. He is wrong.

Okay here is the POOP AGAIN on synthetics:

Good quality Synthetics are perfectly fine to use in a normally operating (read still with factory OMP) rotary. The quality ones like Royal Purple, Neo, Redline and Mobil 1 all burn at under 500F with a low low ash (ash is what is left over after it burns) content (in fact lower than many cheaper conventional oils like Havoline, Kendal, Penzoil, etc).

Quality Synthetics as mentioned above are perfectly fine to use after the engine has passed its break in period and will not add or create deposits anymore than a good quart of Castrol.

Lets compare a couple 20W50s real quick:

AMSOIL 36 482 -38 <.5 ---
Castrol GTX 122 440 -15 .85 .12
Red Line 150 503 -49 --- ---
Kendall GT-1 129 390 -25 1.0 .16
Valvoline Syn 146 465 -40 <1.5 .12
Valvoline Tur 140 440 -10 .99 .13

The first number is Viscosity, how well it pours cold at it's temp range (lower the better)

2nd number is Flash, what temp the oils burn at (ideally between 400F to 500F, but not lower than 400F.

3rd number is Pour. Pour is tested 5 degrees F above the point at which a chilled oil shows no movement at the surface for 5 seconds when inclined. Again lower is better.

4th number is Ash. what is left after the oil is burned, again lower numbers are best as high numbers will lead to coking and build up in our engines.

and 5th is Zinc content. Zinc is used as an anti-wear additive, but in too high amounts it can cause build up. .11 is generally considered the highest you need for a 5000 mile drain interval and .13 the highest you need for a 7000 mile drain interval.

You can see just from those numbers that the Synthetics are every bit as good as conventional oil and in some cases better. But they all burn way under the combustion chamber temps found on our cars.

The scary thing is oils like the Kendal GT1 and Valvoline Synthetic, both of which have a high build up potential as well as the Kendal doesn't even meet the 400F min for flash meaning that the oil will break down radically fast in our cars and leave junk behind when it does.

But cheap synthetics (just like the Valvoline above) will leave ash just as bad if not worse than cheap conventional oils (again as mentioned above).

Avoid cheap mixes and cheap oils period!

Now the advantages, it is not unusual to gain about 1%(sometimes 2%) HP increase using a good synthetic (Royal Purple, Neo, Redline and Mobil 1).

Down side, cost! Typically the average Synthetic costs about $35 a half case (enough to change the oil in our cars), vs the $15-$20 a case that Castrol GTX is generally found for. So the quality synthetic is about 3 to 4 times more expensive.

IMO if you have money to burn or you are racing and need that extra 2-3 HP go with the quality Synthetic. Again it is perfectly safe. Just avoid the crappy/cheap ones (like Valvoline, Kendal, Havoline, Etc). But remember just because the bottle says you can go 10K miles on a synthetic you can't. They need to be changed just as often as conventional oils.

But if you are like the rest of us, and money is an issue, then use the Castrol or Penzoil and change your oil reguarlly.