What Oil Weight Should I Use
 

I have finally finished my short block, and after getting my hands on some (very hard to find) Volvo springs I am almost ready to install the head, as soon as I can take it back to the machine shop to get the springs installed. I've been having trouble figuring out what oil weight I should use, both for break in and once the motor is running boost. These are the components I am using, paired with an EFR 6758 (IWG 0.64A/R) and Kraken turbo manifold/DP to make in the neighborhood of 350whp with 15-20PSI on E85, maybe closer to 400whp depending on how things turn out. I am working out the intercooler situation but I am still a long ways from that so it is not really a priority right now. I am using a Getrag 260 for the transmission (ACT BM11-HDG6 clutch) and the 3.9 Torsen already in my car, welded ear and KMiata PPF delete.

Supertech 9.5:1cr 84mm Pistons
Wiseco NPR XX Rings (0.017" top end gap, 0.020" second end gap)
Eagle Rods
ACL STD Size Race Bearings (0.0022" main clearance, 0.0025" rod clearance)
Boundary S2 72 PSI (2-shim) Oil Pump
ARP Head and Main Studs
ATI Damper
Cometic 84mm .040" Head Gasket
Volvo 418737/VS855 Valve Springs (Otherwise stock rebuilt BP6D head)
Gates 42134 Water Pump
Supermiata QMAX Coolant Reroute
Mishimoto Radiator
Fueling - 1000cc DW Injectors, Walboro 255lph, Radium Rail, Fuelab 515 FPR (return style routing), GM Flex Fuel Sensor
Ignition - Sadfab LS Coil Bracket and AC Delco D585 Coils (wasted spark), NGK 4644 Plugs
And an MSLabs MS3 Mini ECU

I have a feeling I cannot use 5W-30 or 10W-30 anymore as my bearings clearances are larger than OEM spec, although still well within the serviceable limit. I do have a higher pressure oil pump now, but then again I'm not experienced enough with the science behind oil weights to make the call, the guys on miata.net didn't have an answer so I am coming back here to get some more relevant opinions. I live in Houston and even in the winter it barely touches 35F.

PFA
https://cimg1.ibsrv.net/gimg/www.mia...688a058fb6.jpg

Assuming no catalytic converter and a desire for reliability/ service life over peak power, I would recommend race oil. Good race oil has enough zinc to kill a catalyst but helps keep bearings happy. Unless you plan on a lot of starts below 35° or a tendency to add a lot of load before oil gets above 175°, no need for viscosity below 15w. So 15w50 or 20w50 race oil. We use Amsoil Dominator 15w50 in our race engines but there are several good alternatives.

If its a dyno queen, 0w20 street oil, wear good eye protection and have a fire bottle ready ;)

Thanks for the answer, this is the first I've heard of racing oil (quite expensive at $16 a quart) but it looks to provide some real benefits. I am not running cats, but I do see on the product page that it is not recommended for long term use in passenger cars and light trucks. Is this warning because of the damage it causes to catalytic converters? If so, can I safely disregard this warning?

I've been rockin' the 15w40 rotella t6 in mine.. seems pretty happy with it. much better oil pressure than the 10w30 oil I broke it in with.. I'll switch to 5w40 in the winter.

Same here, 15w40 t6 with built motor with similar clearances to OP

Much better availability as well, I can get this stuff at almost any parts store versus the Dominator oil.

I guess the general consensus is to use a 40 to 50 weight oil instead of 30 like OEM (with my clearances). Since I live in Houston I'll use 15W during the summer and 5W during the "winter". Just the answers I was looking for, thanks a bunch guys.

I also run rotella

Good video on Redlines difference in the PCMO and race oils:

You gave lots of information, except the most important part.
Are you running street or roadcourse? Anticipated oil temperature?
Huge difference between street/autocross/drag use at 190*-220* and hot roadcourse at 270*-320*.

Good Oil article
https://grassrootsmotorsports.com/ar...il-track-and-/

Road course does not inherently mean high temps. My S1 (220whp) wheel to wheel cars sits between 210-230° with a large Setrab core.

So the guys using 15-40 are all using oil for diesel engines? Besides being a cat killer is there any other concern?

Rotella T6 is one of the most commonly recommended oils on here for any cars. I don't believe it is a cat killer. My roommate runs it in his stock NA and has no issues. He swears by it and has been using it since he got his car 4+ years ago, around when this thread was originally posted.

Nothing magic about rotella t6, but it is a good oil at a cheap price. It's a real full synthetic that handles the heat from turbo cars well. It used to have lots of zddp, but recent formulas have changed that, though it still has more than most gas engine oils. I run it on my cars, with the principal that I can change it more frequently since it's so cheap.