Originally Posted By: ekpolk
Originally Posted By: BuickGN
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What odd sounds? By the time it starts making sounds other than lifter tick, it's too late. If this were the other way around and the guy had put a 30wt into a 20wt spec'd car, the world would be coming to an end.
Surprise, surprise, I disagree!
As you may recall, I owned an 04 G35 which of course had an earlier version of this engine, the VQ35DE. The oil recs for that engine were 5w-30, 10w-30, or 10w-40. Although I experimented around some early on, I mostly used the 12 cSt GC in this engine. Of all the cars I've owned, this one's engine sent the most signals concerning oil variables. To my ear, it was noticeably harsher sounding on thinner oils, and smoother with heavier ones. Mine also exhibited timing chain rattle on startup. The various TSBs claim that TCR only happens in certain VQs, mine was not in the nominal window for it, but even the dealer agreed that it had TCR. But very strangely, only on thin 30s. I say strangely because TCR is supposedly caused when full oil pressure does not reach the timing chain tensioner (timing chains vs belts is not all good...) quickly enough. You'd think the thin oils would "get there" faster, but something else must be happening.
One of the obvious problems here is that this sort of "analysis" relies invariably upon a very, very subjective element -- how the engine sounds to the owner (yes, I know that everyone out there who has worked in a dealer service shop is cringing). For me, anyway, the VQ is an engine that really does talk to you, and really will tell you different things when fed different oils.
Now, without doubt, the VQ40 truck engine is tuned very differently than the VQ35 used in various sports sedans. I would just encourage the OP to listen carefully and see if he can discern any sound differences with the thin oils, and if so, react as he sees fit.
And I thought you were going to agree. Then the world really would be coming to an end lol.
This is more in response to your previous post but to your statement that it's generally accepted to move up or down one viscosity.
I agree in most situations. However, some 20wts put you dangerously close to the accepted minimum HTHS values for acceptable wear. Going from a 40wt to a 30wt does not. If the engine recommended 20wt from the factory, it's likely it doesn't require as high of HTHS.
