Considering I have a pan or rear main leak will the pure syn oil leak worse than syn/blend? I thought I read somewhere on the forum from another that is what happened to them. I guess I could give it a try and see what happens?
Science time on why *SOME* synthetics cause leaks, or how that may have started
tl;dr early synthetics didn't play nice with old school gaskets. Since then, material compositions and understanding has changed such that it's a non issue. The notion that synthetics cause leaks was never a myth, but you need to look at it with a relevance to the time.
There are 3 base oil types that can be used and be marketed as a fully synthetic oil
1. Hydrocracked - Basically ultra refined conventional oils such that the molecular weight is homogenous and offers the advantages of synthetic. Most cost effective
2. PAO - Polyalphaolefin - longest service life, resistant to breaking down, fuel dilution, water contamination.
3. Esters - Best wear protection, does not play nice with short trips or other scenarios where water contamination is a concern. "sticks" to metals on a molecular level. Also innately acts as a detergent as is extremely effective at breaking down sludge/varnish.
Now onto the leak bit: Esters cause seals to soften slightly by the nature of the molecule. Redline is notorious for using esters in their oils. May also explain why folks with 30+ year old cars say to use redline in the transmission and don't have any issues w/ leakage. It's a group V base stock - the most recent one
Hydrocracked oils, for all intents and purpose, behave as would a traditional dino oil. If you're looking for a 1:1 swap over to gain the benefits of synthetic, this would be your best bet. Rotella uses it in their synthetic lines, and it's fairly common to see house brand synthetics use this base stock.
PAOs: The group IV base stock. These provide the longest service life - Amsoil signature series, mobile 1 annual, etc. However, this particular base stock causes seals to shrink slightly. In the early days of synthetics, this wasn't quite understood. So you'd fill your crank case up w/ a good quality synthetic oil only to find it leaks like a siv. What gives??? Well, your seals shrunk. That's since been corrected and manufacturers have a good grasp on it now, and add some seal conditioners to the oil to counteract this property. Ever hear of anyone recently switching to synthetic and having their seals shit out? No? That's why. Just took some development. These also didn't play nice with the old school cork gaskets and started making rounds during the transitional period when we were moving to more modern nitrile gaskets. So that could have played a big part in the whole leak issue.
Also of note: This same relevance is why I hold the personal belief that 5w30 is an out dated oil weight and 0w30 should be used instead. We started using SAE30, because the 10w30 of the time would shear down to it's 10 weight base stock rather quickly. The same happened when 5w30 came around. Vehicles stuck with a 10w30 recommendation for years after until the tech matured and we came up with better viscosity modifiers. And now we're at the point were 0w30 provides much more desirable cold flow characteristics while still maintaining the appropriate viscosity at operating temperature. That has matured for long enough that shear down is a non issue for even extended oil change intervals.
If you don't know, 5w30 is composted of a 5 weight base stock with viscosity modifiers added such that it's an effective 30 weight oil at operating temperature. What you end up with is an oil that's thinner than an SAE30 when cold, but still maintains the appropriate viscosity when hot. The lower the first number (15, 10, 5, 0, etc) the lower the viscosity/temperature curve. i.e. the less the viscosity will change across a temperature range.
