penrite syn is not fully synthetic, but group III mineral oil. Just for your information.
Just use a quality oil, not cheap and not Castrol. 10w50 is fine, realistically a minimum. Most people who change to quality 20w50 say their engine is quieter at startup. Consider quality oil offers boundary lubrication at startup before you reach operating pressure- and dont go driving hard untill its warm please. The difference- you can create a higher shear resistance/ load caryying ability in a higher weight oil, and the closer the numbers, the less the oil relies on polymers to meet the high temp figure. These polymers are generally the first part of the oil to wear first, and dont offer any additional lubricity.
I'm using 25w60, like a few people I know in an ecotec- which is well known for upper cylinder noise, and it is quieter in startup than ever before (except for the starter motor bearing ****ting itself) which has been replicated in a couple of Gen III's. So low temp weight isnt the entire story.
I would consider, more than anything, something that is rated S_ and C_ (ie for diesel use as well) as this simply means it is a high detergent oil, and i would say more likely than not you have some carbon build up in oil galleries and lifters that may be contributing to the noise, unless you have specifically removed this build up by using high detergent oil or additive or mechanical means (removing and cleaning)
Just for some thought......geez i can talk a lot!
and i did clean alot of gunk out of the inlet valley and upper cylinder bits...