what weight of oil should i use

[gravismaximus]

what weight of oil should i use---- at the end of last summer when i started my 81 which has a rebuilt mild racing 350. i could hear a knock from the lower end i belive, but as soon as it would start to warm up (100f) it would go away. scince i rebuilt it i have been using vavoline synpower (full synthetic) 5w-40. seeing how oil is a double viscosity and trys to thicken as it gets hotter, shouldnt i go to 10w-40 or maybe even 15w-50. so its thicker to start fof with so i dont get the knock. your thoughts please.....

 

[Vettefan87]

In my C4 I run Castrol 10w30. Once I get my motor broken in I will run Mobil1 with the same weight. This is the weight the rebuilder recommended I should run, and a lot of others here have said that too. I have heard no bearing knock or any knock at all. I know that the C4's were pretty much designed for the usage of 10w30, but I am not sure about your 81.

 

[lnirenberg]

Not to change the thread, just a mild redirection--my car has less than 10,000 miles on a rebuild plus new higher lift roller rockers. I have no idea the type of oil that has been used historically but I am considering going to full synthetic. Any upside downside to synthetic in older cars?

 

[gravismaximus]

well after engine brake in, changing is a bad idea at first. the synthtic will leak out of gaskets and go to different ares nomal oil didnt go. i recomend doing a syn blend. but can somone answer my original question, thanks

 

[Vettehead Mikey]

Use whatever your owners manual recommends. Trying to second guess the manufacturer is usually a futile game.

 

[wishuwerehere82]

10W30 synthetic should be sufficient in the NY temperature range. if you were colder the 5W30 would be appropriate. Hotter climates would be good for 15W50.

Try the 10W30 and if it doesn't fix the knocking problem,then you should try to identify where the knock is coming from and see about getting it diagnosed.

 

[Vettefan87]

If it is the main bearings then I would have to say that they were already scored. That was the the case in the last two rebuilds of my motor.

 

[TWINRAY]

what weight of oil should i use---- at the end of last summer when i started my 81 which has a rebuilt mild racing 350. i could hear a knock from the lower end i belive, but as soon as it would start to warm up (100f) it would go away. scince i rebuilt it i have been using vavoline synpower (full synthetic) 5w-40. seeing how oil is a double viscosity and trys to thicken as it gets hotter, shouldnt i go to 10w-40 or maybe even 15w-50. so its thicker to start fof with so i dont get the knock. your thoughts please.....

Off the wall - could it be a lifter noice? Someone once tried to convince me I had a knock in my '69. It went away when warm. I adjusted the lifters and found out it was one lifter that needed adjustment. As far as oil, my .02 is to try to avoid the ones that have/need a lot of visocity index improvers to get more than a 20 wt swing ie: 10w30 is better than 10w40