I have a Chevy Silverado with a 350....
The guy that owned the truck originally used Amsoil
The guy I bought the truck from used Valvoline 10W30....he owned the truck for 2 years before I bought it. It currently has 240,000 miles on it.
It dosnt smoke...it dosnt use oil...

I think Amsoil is the best...even though I am still changing with regular oil...I believe the secret to my lack of engine problems is the result of the Amsoil.

 

i'll just fall back to the ol'

89 E-150, 300 I-6 with the only 5 speed stick i have ever seen in a van (from the factory too), 4:11's and coil overs in the back too

 

i just past 10k on my 289 and i have been considering mobil one but i don't want leaks

 

i use motorcraft oil in everything its cheap and works well

 

You just cannot beat GTX 20-50. I would only run syn. in a late model smog motor that runs much hotter than a 289.

 

Synthetic oil performs admirably in heat, but also offers many benefits in extreme cold. Petroleum-based motor oil thickens in cold temperatures, requiring the starter and battery to work much harder to start a cold engine. Synthetic oil is not as affected by low temperatures, and it will flow much easier at engine start-up. As an added benefit, the cold temperature properties allow it to be quickly pumped throughout the engine, offering much improved start-up protection against friction.
Quite frankly, I can think of no disadvantages to using synthetic oil except for the price.

 

Glenhow much did the seals leak when you made the change it is just my car is in somebody else garage and lots of oil leaks ain't very nice for them. So if i do change i want to be prepared forany oil leakage.

 

I haven't changed to synthetic in the 65. I run Mobil 1 in the 03 Mach1. I am going to change to mobil 1 in the 65 next week, so will let you know how much it leaks. My understanding is that they leak for awhile, then eventually stop leaking. We shall see. I have been using Rotella in the 65 which I believe to be the best dino oil.

 

I live in S.C so cold is NOT a concern. I know of 2 engines (gas) with over 1,000,000 miles on them and have had no rebuilds and are owned by the original owners and they used GTX-20/50. There is a BMW R80-RT with 300,000 + on the odometer by the original owner and all he has ever used is GTX-20/50. The ONLY disadvantage I see to the synthetic is the PRICE. The reason most manufactures of high performance cars use it is for the heat factor or like GM to .2-.3 mpg increase that is worth about $200,000,000.00 in engineering costs saved. I use 15/50 Mobile-1 in both my 95 Z/28 and the wifes Honda and the BMW R1150-RT but I use GTX in the 65 289 and the cast-iron 1977 XLCH Sportster.
AT&T uses the cheapest 5-30 they can find in our trucks and my 99 Chevy 4.3 Vortec went 148,000 with no problems. Only a failed water pump caused a blown head gasket and they put a new motor in but the compression and ring seal on the other 5 cylinders was within new specs.

 

On this same subject about leaks..I first used Mobil-1 in a 1979 Fiesta with 100,000 on the odometer, had no leaks and went 12,000 miles without having to add any oil. The car had only seen GTX-20/50. The leaks occur when your oil cokes-up on the surface and then the seals dry out, shrink and develop microscopic cracks. When you put in Mobil-1 it cleans the crud off the surface and then it leaks out the cracks in the seals.