unless the lifters are shot do NOT replace them or mix them... you will very likely kill the cam... and IF the lifters are shot then the cam is likely not in very good shape either... always replace both cam and lifters

 

check your oil frequently when you switch, sometimes those old motor just cant keep synthetic sealed!

 

What if I use a thicker grade synthetic oil ?
Does synthetic really help older motors ?

 

Karioth, I was referring to the process of pulling the intake, valve covers, oil pan, etc. and rinsing the interior of the engine with diesel manually, not running an engine flush product thru it. If I were going to go thru that much trouble, I'd rather have the engine on a stand in a clean shop where I could do a good job at it than to try to work across fenders, etc. , plus I'd probably put a new coat of Chevy orange on it, too
IMHO, you'll add enough dirt and crud while you're trying to wash the interior with diesel as described with the engine in the car that you may actually end up in worse shape when you're finished than when you started.

 

There are many myths and some truths circulating regarding synthetic oils and seal leakage. Due to the significant variation in synthetic oil base stock, blanket statements about synthetic oils and seal leakage may be inappropriate. Many synthetics have a highly processed petroleum basestock, so the basestocks effects on your seals should not change. Others such as AMSOIL and Mobil 1 have a PAO basestock, which tends to shrink seals. And yet others, such as Redline use an ester basestock which tends to swell seals. Most synthetics contain higher levels of detergents, which will clean sludge from seals exposing them to oil. If they are dried and cracked due to sludge buildup, this could lead to leaks.

If you have a clean engine and you are experiencing no leaks, then you should not have any leaks with AMSOIL or other quality synthetics. On the other hand, if the engine contains sludge due to age or inappropriate maintenance practices, you may experience a temporary leak. When the seals have gone for sometime without oil contacting and nourishing the seals due to blockage from sludge, they will harden and shrink. Since most synthetics, including AMSOIL, contain a significant amount of detergents, they will clean and remove this sludge exposing dried out seals to the oil. Depending on the synthetic oil used, this leakage may or may not be temporary. AMSOIL uses other additives such as Ester which will improve the seal’s suppleness and counters the shrinkage of the PAO basestock Based on reports I've read, AMSOIL should swell the seal 3% to 5%. But, if the seal has a mechanical fault, such as a crack or tear, then AMSOIL can't repair the damage. How other synthetics effect the seals is highly dependent on their basestock.

 

Subdriver... thanks!!
This was a load of helpful info.

 

I don't want to start an arguement here but I really don't see the logic here. A new lifter is flat on the bottom and has no wear pattern at all. The cam lobe will be slightly crowned and would touch the lifter in the center and create a new pattern.
Anyway, it wouldn't be much more work to change the cam too.
My point was, don't run any of that crap thru your motor...engine running. Pour it thru and let it drain out probably would be ok but the best thing would be to change the oil and filter real often.

 

Change your oil and filter add 1 cup atf to carb while engine is running repeat before next oil change and should clean up fairly well by 3rd oil change with atf.

 

As you would expect, the lifter has formed a pattern on the bottom congruent with the pattern on the cam lobe, which mates the two as a match. However if the cam and lifter setup is relatively new, then you may get away with it as the wear pattern has not set in yet.

Adding a new lifter to an existing cam lobe that does not have the corresponding wear pattern is asking for trouble...and the OP mentioned the lifters/cam have been together for some time apprently... that is why I mentioned it may be in his favor to change both at the same time.