I am considering a new slant build and was wondering if it is possible to change the bearing clearances to safely run a thinner oil, or if this even makes sense. My thought is less drag from thinner oil = more power. Also, my blower setup will run better with thinner oil. I would prefer a low viscosity synthetic.
Thanks in advance for the suggestions.

Yep, build your engine with bearing clearances at the small side of spec, then pour in your synth 5W20 and off you go.

See most of this thread (but put on your fireproof suit first; about engine oil a lot of people have sincerely-held religious beliefs).

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I know engine oil is a touchy subject with very strong opinions on either side, but the synthetic has proven itself to me in several different vehicles. The only one that didn't like it was my 96 Dodge Cummins. It had pressure and never broke, but made a loud racket to the point of having to switch back to conventional.
I will check with my machine shop on clearances and if we will need to restrict or open up any other areas. If anyone has additional info on things to do to the slant specifically, I am open to suggestions.
Thanks

If you build it with tight tolerances you should be fine with the thinner synthetic. My early years of being a "car guy" I was into air-cooled VWs, and I ran synthetic in them because they exist in a hostile environment. I've used synthetic oil since high school in everything except old daily driver winter beaters that were on their last legs to begin with.

Dan is right, post anything about motor oil and feathers will automatically get ruffled.

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If it was easy, everybody would be doing it.

I probably would only do this for a solid lifter engine....

The manual for my other brand pickup said I could use 5 w 20... and for one oil change I went from 5w30 to 5w20... The oil pressure looked good and it fired up fine after the change, but after about 200 miles of driving the push rods started to clatter (I thought it was a rod knock at first)... After about 500 miles the clatter would start and last a few minutes during warm up, but disappear after I ran it on the street above 1500 rpm for 30 seconds...

After 1000 miles I changed it back to 5 W 30 and had not problem....


My last drag engine I set for the tight end of the bearing spectrum, and ran 10W 30 like I always did, got the same oil pressure as another engine I built that was middle of the road but a nudge on the loose side... so tolerences didn't make a difference.

H'mm. I'd have an opposite opinion; I think a hydro-lifter \6 built to tight tolerances would work very well with 5W20 oil.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I was thinking only if the lifter bleed down rate was set for that type of oil...otherwise if the lifter didn't pump up correctly at certain rpm then the engine would act like a camshaft that had much less duration and lift than was originally advertised... I know all the Big 3 had tinkered with the idea for a while, and this was even a selling point for the Rhoads Fast Bleed Lifters ("run a race cam, and still have street manners"...).

Would be nice if the hydro lifter data was available from the engineering section of Ma Mopar and see what they found during testing...

Ah, I was thinking of the upstream end of things -- the long and tortuous path the oil has to take to get to the lifters in a hydro \6.
It would be interesting, for sure, but I don't know how much use it would be in answering this present question; 5W20 didn't exist in the mid-late '70s when the hydro valvetrain for the \6 was being engineered.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

It might make a good experiment if the engine is built, to run it on the mech cam for break in and driving, then swap the cam, lifters, pushrods and rocker assembly... and see how the juice cam runs... then if it doesn't work out, swap back to the solid setup....

I see your point and agree it would make for easier top end oiling and getting the lube through the passages and down the pushrods...

Does the "hydro-lifter" slant fill its lifters thru pushrods? If yes the fill-up could take more time compared to conventional fill from lifter bore. Does not need much more than a bit worn lifter doea not keep pressure during engine shut period and the rattle exists. Cheapest fix is to use oil which "sounds" best.

I had to took lifters twice out from Harley and fill them again with tricks like medical syringe. Sometimes they wont fill up with any reasonable rime if they have air on them.

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1966 Dart. 170 cu.in. 200 rwhp.

FYI- the slant six hydraulic lifter only needs 3-4 PSI to "pump up."