I've used a lot of hi-temp silicone and never had a problem with it . . . but I learned a long time ago, there are places to use and places not to use it. I use it only on paper gaskets, and then I rub it into the paper to sort of "rubberize" it. It will help keep oil from weeping through the paper. I 110% agree with Ret.Bugtech . . . it's great stuff, but there are places and ways you can use it and do more harm than good. My advice is, use it ONLY if you really need it.
My best example of it doing harm has nothing to do with VWs. I've seen the blind bolt holes on the side covers of Sulzer Weaving Machines busted out cause someone used half a tube of silicone to keep the cover from leaking. After enough silicone builds up in the bottoms of the holes, the pressure created when you run the screw down against it will bust a chunk of the casting out of the bottom of the hole. Once you bust out those holes, oil WILL leak out the treads of the bolts until you replace the $6000 picking housing casting (probably a 2 day job for 2 techs. plus two days of lost production). The chunk of cast iron that popped out will also drop down in the oil break in the bottom of the picking housing and can wipe out $2000 worth of fast moving, precision parts. I ran a job at work with 42 of these machines on it for 4 1/2 years. On average I probalby had to pull off a side cover 2 or 3 times per shift. In all that time, I NEVER had a problem sealing the cover without silicone. People put it on there to keep it from leaking . . . but if you did it the way the taught us at Sulzer school, you didn't need it to keep it from leaking. So why use it? And if you're going to use it, WHY USE SO MUCH? The average Joe thinks, if a lot squishes out from between the parts it'll be sealed better. Not true. The stuff that DIDN'T squish out is what did the sealing, the stuff that squishes out it how much too much was used . . . and the amount that squished out to the outside so you can see it and gives you that "warm fuzzy it's not gon'na leak now" feeling . . . there's that much more INSIDE of whatever you tried to seal. When a chunk of it breaks loose, it might actually seal off something . . . something like maybe an oil galley? I rather deal with drips in the driveway.
So, on the pushrod tube seals . . . Make sure the sealling area on the block is CLEAN. Make sure to push rod tubes are straight and the correct length. Make sure the seals and tubes are centered correctly when you start torquing the heads down. Torque the heads down correctly. Do it right and they won't leak for many many moons. If you're talking 'bout a type 4 engine, silicone makes even less sense, 'cause you can change the seals without pull the engine.
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