Is synthetic bad on fresh motor or something? I was more concerned with low viscosity
Assuming you got new rings with a fresh hone... The rings need to seal. Synthetic is too slippery to allow the rings to seal properly with the cylinder bore. It wont blow up or anything with synthetic... you may just end up with blow by... or a lot of blow by lol.
And i always add a bottle of Comp ZDDP additive to help protect the cam. (If flat tappet i would add at every oil change. If roller cam just using a bottle at break in is good enough).
If you havent broke it in yet, use the cheapest 10w30 you can get with a bottle of the Comp ZDDP or an equivalent. If its a new flat tappet cam fire it up and go right to 2000rpm and vary up to 2,500 for 30mins. If roller cam or flat tappet is already broken in with the lifters... skip the cam break in and go right to engine break in... fire it up and let it get up to temp. Usually rev it up a few times and make sure there are no leaks and set timing. Let it cool down. Then bring it up to temp and cool down 2 more times for a total of 3. Change oil and oil filter. Use your better oil now... typicall weights i see are 10w40... 15w40... 20w50... ect. Here in florida you can use a straight 40 or 50 weight. If the engine builder setup tight tolerances like the factory you can run a 30 weight but typically its 40 or 50 for rebuilds.
Usually i would go straight to the track or dyno at that point... but if its a daily driver or weekend cruiser and longevity is the goal... go easy for the first 500 to 1000 miles. Put it in varying loads, speeds, rpm ect. For the first couple miles you can do some engine braking to help seal the rings better. Put it in first, speed up to 4500 or so and let off. Let the engine slow you down. You can do this while driving in D as well. As you’re coming up to a light instead of braking pull the shifter down to 3rd them 2nd ect again letting ur engine slow you down.
After the first 500 - 1000 miles change the oil and oil filter again. After 2-3k miles then you can switch to synthetic if you like.
Ive never heard of an engine builder telling someone to use full synthetic on a rebuild. The only case i could see is if they used the same rings with no new hone on the cylinders (which usually is not the case)
Also ONLY use Wix or K&N oil filters... they are the best (smallest micron) filtering compared to the other off the shelf brands like STP or the absolute worst... Fram. There’s a local engine builder that has the best reputation in Central Florida and they won’t warrantee your motor if you use a fram filter for break in lol.